Manchester United, Barcelona, AC Milan and Juventus.

They're all Champions League winners, but what else do they have in common? 

Aside from the fact they were in the 'Super League 12' that fell flat on their faces earlier this year, they were all defeated at Celtic Park over a nine-year period from 2003 until 2012.

The stadium had a reputation as a lions' den and many challengers left with red faces and burst eardrums.

Inter Milan and Manchester City have fairly recently taken off from Glasgow airport half-breathing sighs of relief after 3-3 draws in Glasgow's East End, but those days being Parkhead's peak over the last decade or so tells it's own story.

Under Martin O'Neill and Gordon Strachan, Celtic competed with, and defeated, Europe's best. They looked them in the eye and, backed by 60,000 supporters, said "you and what army?" to the elite.

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Those days are disappearing further in the rear-view mirror and it doesn't seem like they're just round the corner again either.

Celtic are now a club for whom Champions League qualification is still a target, but just as unlikely as it is likely. Europa League involvement is more conceivable these days.

Ronny tried to roar, Brendan wanted intimidation and Neil Lennon spoke of thunder. One clap reverberated around the continent after a 2-1 win against Barcelona in his first tenure, but lightning has struck more than once in the Champions League qualifying rounds since then.

Malmo, NK Maribor, AEK Athens, CFR Cluj and Ferencvaros have all overcome Celtic prior to the tournament's group stage and many expect Danish side FC Midtjylland to do the same starting this Tuesday.

It's five weeks to the day since Celtic's prolonged effort to appoint a manager concluded with the ignition of Ange Postecoglou's 12-month rolling deal and three pre-season friendlies have been played in those 35 days.

Experimental and youthful Celtic sides tasted victory against Sheffield Wednesday and Charlton Athletic before a stronger XI drew 0-0 with Bristol City.

With Vasilis Barkas, Ismaila Soro, Karamoko Dembele and Mikey Johnston all substituted with injuries, Kris Ajer departing to Brentford and Ryan Christie and Odsonne Edouard suffering "niggling" injuries, or being wrapped in cotton wool ahead of potential sales, it's anyone's guess as to what side will take on Midtjylland next week and be the tools Postecoglou will try to work a miracle with.

We'll probably get a good idea based on the team he chooses to play Preston North End this Saturday.

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Working on the hypothesis the above aren't involved and Barkas' finger injury sustained in the warm-up rules him out, Scott Bain will start in goals, the sight of which is nothing new for Celtic fans given he's done it 70 times. 

Anthony Ralston is the club's sole natural right-back as it stands and will continue in that position, but it may just be in the short-term with the club linked with both of Rennes, Sacha Boey and Brandon Soppy.

Stephen Welsh is now ingrained in the first-team mosaic and with Christopher Julien still on the road to recovery after a knee injury and Kris Ajer wearing a Brentford training kit by that point, it could be new signing Osaze Urhoghide alongside him. 

Urhoghide can also play right-back and if Ruben Kazan centre-half Carl Starfelt can somehow be shoehorned past Covid red tape in the nick of time or Nir Bitton is available, one of them could also partner Welsh.

Callum McGregor and David Turnbull are sure to strut their stuff in the middle but with Soro and Tom Rogic's fitness uncertain, Liam Shaw and Ewan Henderson fairly inexperienced and Luca Connell yet to play a competitive game for the club, there aren't any others who've experienced such occasions and are available to play.

Hold on a minute... Olivier Ntcham?

The Frenchman has reportedly told Postecoglou he wants to stay at the club this season with seeming escape routes elsewhere in Europe proving to be roads to nowhere. A midfield of him, McGregor and Turnbull is capable of dictating a game but will probably require McGregor to act as a solo pivot.

On the wing, Postecoglou may well have to, well, just wing it.

James Forrest didn't travel to Wales for the pre-season training camp because of self-isolation requirements, while Mikey Johnston's hamstring injury probably rules him out. Karamoko Dembele, of whom the manager is a big fan and doesn't want to "protect", was scythed down against Bristol City and clarity is needed on the consequence.

If Marian Shved plays, we might as well stop speculation for good.

Ryan Christie, who has started on the right side for club and country before, is entering the embers of his contract and it remains to be seen if Celtic will risk playing him as injury could prevent collecting a fee from Crystal Palace, Monaco, Nice, Leeds or whomever before smashing a bottle on the side of Good Ship Christie and waving him on his way.

Interestingly, new signing Liel Abada might be given a competitive debut after training with his team-mates for the first time today. The shallow depth of Celtic's options out wide may have led to a debut anyway, but the injuries to Johnston and Dembele could sign and seal it. We'll see if he delivers on the night.

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Alternatively, Owen Moffat has looked solid in attacking midfield and might be given the chance to shine on a serious stage.

Albian Ajeti has been the prioritised striker throughout pre-season and the smart money is probably on him to start against the Danes but again, it depends on the club's intention with Odsonne Edouard.

He has played, and scored, during the training games but was missing against Bristol City on Wednesday with a "knock", so it's not out the question he's being kept safe for an eventual move to pastures new.

With Leigh Griffiths playing no part in Celtic's pre-season tour while a police investigaion took place into alleged, and subsequently not found, criminal behaviour, it's a huge long shot he'll be given the responsibility of leading the attack.

The potential starting line-up has a huge question mark hanging over it and we might get a better idea of Postecoglou's intentions after the friendly against Preston tomorrow.

One thing is for certain though, more reinforcements to add to today's signing Kyogo Furuhashi are needed, and fast, if Celtic are to navigate their way through these qualifying stages and not just for the Champions League, but it's second-fiddle sister competition too.