He was invited along to a Celtic first team training session in 2014 to supposedly make up the numbers.

Five years later the Hoops were crunching figures worth a mind-boggling £25 million.

Amazingly the most expensive footballer in Scottish football history was discovered largely by accident than design.

The rise and rise of Arsenal and Scotland defender Kieran Tierney’s football career has been nothing short of meteoric.

Back in 2014, a solitary Celtic first team training session was all it took for then Hoops managerial duo Ronny Deila and John Collins to realise that they had unearthed a potential star in the making.

What happened next is fairytale stuff.

Incredibly Tierney was FOURTH choice left back for Celtic U20s at the time and above him in the pecking order were Joe Chalmers, Calum Waters and Aidan McIlduff.

How did Tierney parachute in to the Celtic first team from nowhere?

How did the youngster with the precocious talent oust every one of his contemporaries and go on to displace Emilio Izaguirre in the left back position as well as become a first team regular and firm Hoops fans favourite?

Former Celtic, AS Monaco, Fulham and Scotland midfielder John Collins takes up the story.

The ex-Celtic assistant said: “We needed an extra body to join in Celtic first team training.

“Joe Chalmers, Calum Waters and Aidan McIlduff had all gone away to play in an U20s game and Kieran Tierney was left behind.

“We always watched the reserves playing and we were well aware that Joe, Calum and Aidan McIlduff were ahead of KT in the pecking order at Celtic.

“That’s part of the job to watch all the sides and KT was younger than all three of them.

“I am one of those people who believe that age means nothing…absolutely nothing.

“We are involved in professional sport and the only questions that need answered are quality and ability.

“Some people talk about age but the younger the better as far as I am concerned especially if you possess the ability and talent that KT did.

“We took KT to first team training that day and we could not believe what we were seeing.

“KT immediately caught Ronny’s eye as well as my own.

“He ticked so many boxes.

“He was so hungry, showed a great attitude, he had great application and as you see now that is exactly what he showed in that first team training session.

“KT loved training and he was dedicated to his craft.

“He was flat out every session from the first minute to the last and he was so focused and switched on.

Celtic Way: Kieran Tierney loved trainingKieran Tierney loved training

“From a coaching perspective he was a dream to train and that is what you love on the training pitch.

“Like KT, I got my opportunity at Hibs under the great Pat Stanton at Easter Road and I never looked back after that.

“It is always nice when you can give somebody an opportunity from the academy.

“It is special when a player comes through the ranks and in KT’s case he was also a Celtic supporter.

“He got his chance early on and he took it and went on to become a first team regular.

“Off the pitch he was a good listener.

“That is such a key point in the learning process he was willing to listen at all times to Ronny and myself and improve his game.

“Ronny and I both saw KT’s potential early on and we knew we had a potential star in the making.

“KT was just so hungry for information and education.

“He was so dedicated.

“You just knew KT was something special and he would go on to bigger and greater things from a very early age.

“He had that controlled aggression as a defender.

“His distances were also good and whenever the ball arrived at the wide players feet he was always arriving at the right time.

“He never once let a winger drive on to him.

“That is a great attribute to have for a full-back.

“I don’t think KT was ever out of the Celtic first team once he had established himself in it.

“I always stressed to the young players at Celtic that once they got that coveted first team jersey then they had to earn the right to keep it.

“There is always somebody that wants to take it off you and never take it for granted.”

Speaking on Simon Ferry’s Open Goal show in August 2020, Tierney revealed that luck had played a huge part in his ascension into Deila’s first team.

Incredibly he was contemplating a working life outside of football if he did not make the grade with Celtic.

After impressing then Celtic boss Deila, Tierney’s football life and career trajectory would never be the same again.

The Norwegian gladly took the teenager under his wing.

Tierney said: “I was fourth of fifth choice left back for the U20s.

“Some guys were being offered three-and-four-year contracts.

“I was thinking that I would have to get a job with my dad.

“I got offered a one-year expenses deal.

“I was never going to say not to it.

“All I wanted to do was to play for Celtic.

“When I was 16, it was Aidan McIlduff, who was the left back and he was playing ahead of me.

“He graduated to the U20s whilst I played for the U16s.

“When I was 17, sI went to Lennoxtown and the likes of Joe Chalmers, Calum Waters and Aidan McIlduff were all in and around the first team.

“One day all three of travelled with the U20s to an away game.

“John Collins came and said they needed an extra body to train with the first team.

“I went up and trained and Ronny Deila instantly loved me.

“I was 100 per cent nervous about training with the first-team and I knew I was up against Darnell Fisher at the time.

“He was solid and hard as nails.

“We were really going for it hammer and tongs and I loved that “I was taking people on and crossing the ball over and not letting people get past me.

“Right after the session Ronny came up to me and shook my hand.

“I had a cracking session.

“I had just trained with the Celtic first team and it felt too good to be true at the time and it all seemed like a dream.

“I would train with the Celtic first-team, yet I still wasn’t even playing for the U20s.

“When I first arrived at Lennoxtown nobody really knew who I was. “They knew Aidan because he, Joe and Calum were all playing for the U20s.

“They were all good players.

“I hadn’t really done anything to be playing ahead of them so there was no expectation on me which really helped.”

Tierney’s direct left-back opponents Chalmers and Waters can testify to the fact that he was developing into something of a footballing phenomenon.

Neither man admits to possessing a touch of the green-eyed monster at the fact that Tierney had gained his first team crack at their expense.

Celtic Way: Tierney on the ball for ArsenalTierney on the ball for Arsenal

It is testament to Tierney’s humility that they both remain good mates with the Gunners idol to this day.

They both insist that Tierney has never been afflicted by a ‘big-time Charlie’ attitude.

Tierney is still the same humble guy who pitched up at Lennoxtown with a headful of ambition and even loftier dreams.

The same player who was once pictured walking into Sheffield United’s Bramall Lane for an English Premier League clash armed with his football boots and toiletries tucked into a plastic Tesco carrier bag.

Ayr United star Chalmers admits he knew immediately that ‘action-man’ Tierney was going to leapfrog over them all.

He insists that it was only a matter of time before Tierney started to make his mark with Celtic.

He revealed that even taking part in warm-ups alongside Tierney he was a force of nature.

Chalmers said: “All I could remember back then was that KT was a bit of an action man.

“He was three years younger than I was and he was a bit raw but even during the warm-up he would just get right into it.

“He would come right out the blocks at 100 miles per hour.

“Everybody else would be doing light jogging and KT was sprinting to the max.

“Physically KT was a powerful boy.

“When he trained with the first team all that rawness just seems to disappear overnight and as a player he came on leaps and bounds.

“KT is a boy who we all wanted to do well and succeed.

“His success is our success and there was never a hint of jealousy with the likes of myself, Calum and Aidan.

“At the time when he broke into the first team under Ronny Deila and John Collins, I was out injured and by the time I came back KT was beginning to make his mark, “My contract was also up by then.

“The management team spoke to me and told me they had KT so my first team opportunities would be limited.

“I knew it was my time to leave Celtic as I was never going to displace KT.

“The best thing for me was to move on.

“There was a spell at Celtic where the left back position was a problem.

“Emilio Izaguirre was initially the first choice lost his form and then there was Calum, Aidan and yours truly.

“KT was supposedly the young one coming through behind us but after that first team training session he ended up going above us all and filling that void in the Celtic team “Mentally and physically, KT was just different “He had power and total aggression and he was always on the front foot.

“He was mentally strong as well as tough and nothing ever fazed him.

“Through training with the first team and working with good players and good coaches on a daily basis KT just developed his game massively.

“His passing, his composure just moved on to another level – the top level - when he advanced to the first team.

“That is not to say his passing and composure were poor.

“All the young lads were the same and we all used to think how quickly we could do things.

“KT when he had the chance to cross the ball or shoot was always about the power and he’d smashing it in as hard and fast as he could.

“Over time KT has developed that side of his game and he knows when to do that and not to do it now.

“When you watch him play for Celtic and now Arsenal and Scotland you could see that he has different types of crosses in his armoury now.”

Kilmarnock defender Waters said: “Sometimes in football you need a wee bit of luck and Kieran got his chance and he took it.

“We were never jealous of Kieran.

“Kieran was an absolute inspiration to Joe, Aidan and I most definitely.

“It was more the realisation that when he started to break through to the first team you quickly realised this guy was the real deal.

“I was a year older than Kieran and that was the reason that I was in front of him in the U20s.

“To this day Kieran is a player that I look up to and respect totally.

“You need to be exceptional to make it a club like Celtic.

“It is such a high-pressure environment.

“You need the physicality as well.

“When Kieran started to break into the Celtic first-team environment we noticed how fast he could run and how quick he actually was.

“He had exceptional speed.

“He also worked on his upper-body strength too as has massive legs “I knew when he made that transition up from the U20s to the first team that there would be no stopping him.

“When you train with better players you improve your own game.

“His confidence levels also shot up too as he started to hold his own.

“KT will remember as far back as playing with U11s level alongside myself that at Celtic that you just had to win “If you didn’t win it was a crisis at every level and at any age.

“We just tried to emulate the first team by winning every week and looking back it helped build a winning mentality and a will to win in players like Joe, Aidan, myself and Kieran.

“These are the kind of things that stood Kieran in good stead firstly with Celtic and now Arsenal and Scotland.”

A broken leg sustained on the eve of his competitive Celtic debut in December 2014 could not dampen KT’s fire or enthusiasm.

He would make his Celtic first team debut barely FOUR months later away to Dundee at Dens Park in a 2-1 win in April 2015.

Tierney feared that the Hoops were going to blow a 2-0 lead when he took to the field.

The nine-minute cameo meant the world to him.

Tierney said: “By Christmas I was training with the first-team every day.

“I made the bench against Ross County just after Christmas and then Ronny Deila told me I was starting against Partick Thistle in the February “We trained inside the day before and I have gone into tackle with metal studs on and I have broken my leg.

“The day before I was finally set to make my Celtic first team debut I broke my leg.

“I hadn’t told anybody I was starting.

“I had gone from a massive high to a huge low.

“All I ever wanted to do was play for Celtic.

“I finally came on for my debut against Dundee away.

“I was a nervous wreck when I got the call.

“I came on and we were 2-0 up.

“My shirt was tucked right into my shorts which were pulled up to my neck and I looked like Simon Cowell.

“I was a wee skinny boy who had been plucked out of the crowd just to play for Celtic.

“Dundee scored with a counter-attack to make it 2-1.

“I thought we are going to concede two goals here and I have only played the last ten minutes.

“Thankfully Celtic won 2-1.

“I celebrated by running over to the fans and I went off my nut doing fist-pumps and we had only just scraped past Dundee.

“It felt brilliant.

“I started against St Johnstone a couple of weeks later and the rest as they say is history.”

Tierney’s first Scottish Premiership title would soon follow under Deila and Collins in season 2015/16.

Back-to-back domestic trebles were also plundered under Brendan Rodgers in the next two seasons as KT swept the boards.

He scooped a total of six PFA and SFWA Young Player of the Year awards before the staggering multi-millions move to the Emirates in the summer of 2019.

An FA cup winners medal was also pocketed in his first full season with the Gunners.

Collins is convinced that the seeds of Tierney’s super stardom were sown during those early training sessions with the Hoops first team.

He reckons that Arsenal spotted the exact same qualities in Tierney that both he and Deila recognised in 2014.

Collins is adamant that his mind regularly drifts back to that infamous training day at Lennoxtown.

He derives enormous personal satisfaction from it.

Now he can’t wait for KT to write his next career chapters after taking to the self-proclaimed best league in the world like a duck to water.

Collins said: “From those first training session with the Celtic first team Ronny and I could clearly see that KT loved to get forward.

“He had great energy levels and possessed great pace.

“He had superb bursts of acceleration.

“He really is a smashing player.

“Every team wants a left-footed player and that was also a real asset for KT at the time.

“All clubs look for left-sided players or guys that can play left centre-half, left wing-backs or left wingers.

“We knew that KT possessed all the attributes to be a modern-day defender.

“I am not surprised that he has gone to the English Premier League and not had a problem adapting with Arsenal.

“People were asking how would KT adapt?

“He was another Celtic player coming from Scotland to England and is a big jump up but rather like big Virgil Van Dijk, KT has taken to EPL football like a duck to water.

Celtic Way: Virgil Van Dijk training for LiverpoolVirgil Van Dijk training for Liverpool

“It really has been a piece of cake to him.

“KT had that inner self-belief which all good too players must have.

“That belief in your own ability can carry you a long way.

“At the same time KT has stayed humble and kept working hard and he has continued to do all the things that got him to the top in the first place.

“He will never take anything for granted.

“If you watch KT now he is one of the managers first names on the team sheet and it is the same with Scotland.

“Every time I watch KT, he puts a smile on my face.

“Most coaches will tell you the same thing when they help introduce a young player into the team and they see him going on to have a wonderful career it is always satisfying.

“He is just such a mature and experienced player and has developed into a key player for club and country.

“Arsenal conducted a great piece of business when they splashed out £25 million for KT from Celtic.

“They are a massive football club but at the moment they are not regularly winning the biggest prizes.

“I am sure there will be lots of other clubs looking at KT and admiring him that’s for sure.

“Arsenal are a great club but I think KT just has to focus on where he is right now.

“He shouldn’t look too far ahead into the future as there is always a danger in doing that.

“He just needs to keep training hard and playing well and his career will take care of itself.

“KT’s first ever training session with the Celtic first team will live long in the memory.

“It’s just a nice feeling to think that I may have helped play a small part in the development of KT’s career.

“Kieran Tierney just grasped the nettle and his first team opportunity with both hands when it arrived.

“I wish him every success.”

Whilst Tierney may have had the last laugh career wise Waters joked that he at least has one football story that he can dine out on forever.

Waters said: “It is still a small claim to fame that I once kept KT out the Celtic U20s team.

“Give it another 20 years and that story will grow arms and legs and will enter into football folklore!”

Good luck with that Calum.