CRUCIAL. Critical. Vital. Significant.

Perm one from four if you are Celtic boss Ange Postecoglou.

Or as I am often prone to saying simply because I am old school: multiple choice answer E - all of the above.

Celtic's next three matches have fallen into the season-defining category.

Fixtures certainly don't come much tougher than this stretch, which could invariably make or break their season.

In the next week alone, they will face Hearts at Tynecastle, Dundee United at home and then Rangers at home.

It is a punishing, relentless schedule. Whatever happened to the old adage of taking it one game at a time?

Well, there is no margin or room for error at Celtic.

Every side in the next three-game sequence has taken points from Postecoglou's men during this campaign already.

If Celtic can come through the next three games unscathed then their title credentials and mettle will be there for all to see.

However, the Aussie must feel that he and his team have been cursed this season at times.

Incredibly, on average, it could be argued that six players have been unavailable to Postecoglou every game this season. Astonishing.

Celtic have been significantly weakened after last weekend's 2-1 Scottish Cup fourth-round tie over Alloa at Recreation Park.

That victory came at a huge cost with skipper Callum McGregor suffering a facial knock which is suspected to be a fractured cheekbone that could rule him out to injury for anything between four to six weeks.

Celtic Way:

McGregor is a leader and has worn the armband with aplomb since Scott Brown quit Paradise for Aberdeen in the summer. His presence in the engine room, as it is he who has made the team tick this season, is a serious blow to Celtic's title aspirations.

They are significantly weaker that's for sure but somehow Postecoglou will have to discover a coping mechanism if their league ambitions are to remain firmly on track.

With new striker Daizen Maeda and midfielder Tom Rogic both being called up for international duty and talismanic striker Kyogo Furuhashi still toiling with a hamstring injury and no imminent date scheduled for his return, Postecoglou must feel like the world is conspiring against Celtic.

Only last week momentum felt well and truly with Celtic and there was nothing but positivity emanating from Glasgow's east end.

READ MORE: Matt O'Riley was sold on Celtic switch after one phone call with Ange Postecoglou

Postecoglou had just made his fifth signing of the transfer window in the shape of midfielder Matt O'Riley from MK Dons and both Maeda and compatriot Reo Hatate had made dream debuts for their new club with the former weighing in with a goal against Hibs and the latter turning in a man-of-the-match performance.

Fast forward a week and there is an air of doom and gloom and despondency among some of the Celtic faithful with the realisation that for the next three games at least they will be without the influential quartet of McGregor, Rogic, Kyogo and Maeda.

They are integral to Postecoglou and are surely certain starters for Celtic, all things being equal. The absence of both Kyogo and McGregor earlier this season was a telling factor for the club.

Their respective absences bookended a period when the Hoops won just one match - against Raith Rovers in the League Cup - in a brutal five-game sequence from mid-September to October in all competitions. That stretch included a Premiership loss away to Livingston and a league draw at home to Dundee United.

The Aussie's side has since strung together an impressive 18-game domestic unbeaten run since that stalemate against the Tangerines.

Back on December 14 when Postecoglou was forced to field David Turnbull up front after being denied the services of Kyogo, Giorgos Giakoumakis, Albian Ajeti and Jota due to injury he had this to say in the aftermath of a 1-0 victory over Motherwell.

 “This group of players don’t want excuses, they want to go out there and prove they can overcome every challenge.

"We have had plenty of them since I got here and we will keep tackling them in the same way.

“Through it all, we haven’t made excuses, we haven’t tried to make allowances, we haven’t said give us time.

“We have just got on with it and the squad keeps getting on with it, which as a manager is brilliant to see because it means I have got a really resilient, tough group of players who want to be successful.

“We are getting challenged in a different way now with injuries but I haven’t seen anything in performances to tell me that the players are wilting under any sort of stress.

“If anything, I think they are rolling up their sleeves a little bit further and digging in a little bit more."

It sounds an all too familiar story and pattern this season for Postecoglou and Celtic doesn't it?

The one silver lining and small crumb of comfort this time around for Celtic and Postecoglou is that he at least has one functioning striker at his disposal in the shape of Giakoumakis as well as midfielders Hatate and O'Riley who may now have to step up to the plate much sooner than they might have expected.

READ MORE: Celtic striker Giorgos Giakoumakis must seize his chance in pivotal three-game stretch that could help decide season - Alison McConnell

Greek attacker Giakoumakis netted only his second goal for the club last Saturday against Alloa with a fantastic volley which bore the hallmarks of a predator's finish.

The jury is still out on the £2.5million purchase from VVV Venlo and last season's Eredivisie's top scorer.

As Giakoumakis pointed out after the Alloa match: "I can say the season starts for me from now."

If he manages to score against Hearts, Dundee United and Rangers and Celtic win all three matches then he would buy himself some valuable time, persuade the doubters to get onside as well as make himself a hero in the process.

Not that Postecoglou would need to consult anybody for advice but it might be worth the Aussie having a word in former Celtic manager Gordon Strachan's lug. 

Back in season 2007-08, Celtic faced a similar three-game season-defining sequence.

Within nine days in April, Strachan's men came up against Rangers at home twice with a home clash against Aberdeen sandwiched in between the Glasgow derbies.

Celtic trailed Rangers by six points going into all three encounters but won both the crucial derbies 2-1 and 3-2 and the Reds were despatched by a solitary goal to nil as three consecutive victories were reeled off.

Celtic Way:

In fact, Celtic embarked on a seven game-winning run that culminated in Strachan's men clinching the league flag on an unforgettable night at Tannadice in May.

Prizes aren't handed out in January or February and it ain't over until it's over. Postecoglou will be acutely aware of that.

What is for certain is that the Scottish Premiership picture will become even clearer for Celtic by the end of these three games.

The season doesn't just start now for Giakoumakis. By his own admission, Celtic have been doing their own thing all season under Postecoglou.

The Aussie will hope that come February 2, their title race has not been well and truly run.

However, if Celtic can channel the same spirit of Gordon Strachan's side of 2007-08 and win their next three season-defining matches then they will have 'blown the bloody doors off' in the race for the Scottish Premiership chequered flag.

A lot of people will know that if they do.