How do Celtic replace Callum McGregor for crucial run of games?

But arguably even more significant is that they must also now manage without their captain too.

Callum McGregor is at least as important to the way Celtic want to play as any of the players who fill the roles mentioned above.

He may not score as many as Kyogo Furuhashi, create as much as Tom Rogic, fire in set-pieces as often as David Turnbull or be a dynamic new addition like Daizen Maeda but his influence and fingerprints are all over everything that is good about the Hoops’ play this season.

A hard whack to the face during the Scottish Cup fourth-round win over third-tier Alloa Athletic has robbed Ange Postecoglou of what he has called his “driving force” for the foreseeable future.

Reports vary so far but even the more conservative estimates seem to have the skipper sidelined for four weeks. In Celtic’s hectic schedule, that’s a minimum of seven games and it could well reach double figures if his recovery stretches to six weeks or longer.

In the immediate vicinity, there is the arguably season-defining trio of Premiership matches against Hearts (A) on January 26, Dundee United (H) on January 29 and Rangers (H) on February 2.

Even beyond that, though, there are considerable challenges such as Motherwell and Aberdeen away, Raith Rovers in the Scottish Cup fifth round and the first leg of the Europa Conference League tie against Bodo/Glimt.

While the legitimate strengthening of the midfield in the January window has rightly been met with almost universal praise, is there enough at Postecoglou’s disposal to genuinely fill the substantial gap left by the captain’s absence at such an important time in the season?

In short, probably not. Not directly anyway. But they can manage in other ways. There are options.

This isn’t like earlier in the season, when McGregor missed four matches – during which Celtic won just once: against Raith Rovers in the League Cup – and the team seemed to genuinely struggle to play through the thirds without him.

Well, it’s kind of like it. Kyogo was missing then as well (as part of a five-game absence). So were the likes of James Forrest and Greg Taylor.

In fact, if you count long-term absentee Christopher Jullien and the fringe players yet to truly be first-team players like Karamoko Dembele then Postecoglou has pretty much been without at least four players for each game since his arrival.

READ MORE: Celtic have reached make-or-break time as season-defining week looms large - Tony Haggerty

But, crucially, where the similarities end for this particular spate of unavailability is in the players waiting in the wings to assume the roles.

Reo Hatete's do-it-all quality was on show during his debut against Hibernian while the undoubted energy and bite he carries will become even more important in the central areas as the weeks go on. 

Nir Bitton, too, is going to be key. It's likely it is the Israeli who steps into McGregor's role at the base of midfield for the Hearts game. He plays it differently to the captain, of course, but has undergone something of a career renaissance since returning to the regular starting XI reckoning at the end of October.

It sounds as though Yosuke Ideguchi may well miss some games with the ankle injury he suffered against Alloa but he will presumably return to the fold quicker than McGregor and thus have his own chance to solidify a role in the side with his mix of ball retention and box-to-box mobility.

Barring injury, Rogic should be available again for the trip to Fir Park on February 6 but, in the meantime, new signing Matt O'Riley could well be asked to take on a more important role than perhaps expected in the 8/10 hybrid that he seems suited for.

There's also James McCarthy and Ismaila Soro to call upon if necessary while a switch to the 3-4-1-2 debuted against St Johnstone on Boxing Day, a double pivot, Jota shifting to more central areas in a 4-2-3-1 or even Josip Juranovic in midfield are all options available to the manager, albeit to varying degrees of viability.

Celtic depth in central midfield, with injured or unavailable players in red

What seems the most logical, though, is making use of Bitton's valuable experience to help propel the side's new blood to the fore. At least until Rogic is back anyway.

Indeed, Postecoglou smartly lifted some of the pressure that will accompany the next few games off the shoulders of his new recruits on Monday.

"They don't need to impress," he said. "We've brought them in because we think they'll be comfortable and be able to contribute in our setup. Reo has already had that taste and I'm sure Matt will get his over the next couple of games.

"That's why we've brought them in, we think they can help."

Postecoglou often speaks of bravery and resilience in his teams. It has always, to me, meant the courage to play in the way he asks while having the strength of character to continue doing so even when things are going against them.

Or, as he put it earlier this season: "What we need to do is concentrate and focus every minute rather than thinking somehow the task has become insurmountable".

Collectively, that's what Celtic must do for however long they are forced to survive without their captain. It's all hands on deck.