If variety is the spice of life, Ange Postecoglou must finally be starting to feel like he has his cupboards well-stocked. 

New signings such as Joe Hart, James McCarthy and Josip Juranovic have been recruited to bolster his ranks and, while a couple more are desired, for now the Greek-Australian has what is often termed a 'selection headache' ahead of two massive games in four days.

First up is the Europa League qualifying second leg against Dutch Eredivisie side AZ Alkmaar before a trip to Ibrox beckons on Sunday.

The performances of midfielders David Turnbull, Tom Rogic, Ryan Christie and Callum McGregor have earned plenty of plaudits in recent weeks while, further forward, Kyogo Furuhashi and Liel Abada have introduced themselves to the Celtic faithful in impressive fashion.

Here, the Celtic Way considers Postecoglou's midfield options and how style of play, form and rotation could influence who starts the next two games.

McGregor central to it all

Twenty days ago Celtic banished an initial three-game winless start by defeating FK Jablonec 4-2 in the Czech Republic in the first leg of their Europa League qualifier.

They followed that up with a dominant 6-0 win over Dundee in a match that will be fondly remembered for Kyogo's home-debut hat-trick and arguably Christie's most imperious turn in the green and white.

Celtic haven't looked back since, reeling off another four wins on the bounce and playing some of the most attractive football witnessed at Parkhead in years in the process.

Something else happened in that Dundee game, though. McGregor moved to play deeper as a single pivot. Since then he has been, without meaning to sound too hyperbolic, inspirational. 

It may not be represented in goals (he has none in those five matches) or assists (just the one). It may not even be represented in things like slide tackles (two in five) or aerial duels (one in five). But it works.

Simply put, McGregor as the single pivot works so well as Celtic have so much possession (an average of almost 66 per cent over the past six games). He’s integral to keeping the ball flowing through the thirds and not allowing the side to get bogged down with aimless passes between centre-backs.

READ MORE: The Ange Postecoglou Celtic philosophy that hasn't changed since day one

In front of a defence that is still finding its feet with the technical aspects of Postecoglou's style, it is often the captain that the likes of Carl Starfelt and Hart turn to if free passing options don't seem to be readily available. Whether under pressure or not McGregor happily receives it and rarely looks flustered in doing so.

Having a player with that kind of confidence and composure in front of you cannot be underestimated. In a system that emphasises perpetual motion and purposeful passing, McGregor almost embodies the philosophy.

Ahead of McGregor, Postecoglou has shown a preference for playing Turnbull and Rogic in a dynamic axis of ingenuity tasked with breaking lines, spotting openings in the opposition's resistance and generally causing havoc wherever possible.

After perceived initial struggles under Postecoglou, Turnbull has carved out the kind of purple patch that could heal an injured Barney the Dinosaur. The 22-year-old has five goals across that six-game winning streak and, while he has not registered a direct assist, he does have two second assists and his xA of 2.01 suggests he is contributing creatively as well as finding the net.

Meanwhile, Rogic's languid, jinking style and penchant for producing that unquantifiable attribute we might term 'magic' proved a handful for AZ in the first leg and - as I wrote in my match ratings for that game - offered a problem the Dutch side didn't quite know how to solve.

Celtic Way:

While Turnbull and Rogic are primarily in attendance for their attacking contributions they have recently shown their worth in terms of pressing too. Turnbull, in particular, has stepped up his defensive output in the games against Dundee, AZ and St Mirren.

On the couple of occasions Rogic has not started during the winning run it was Ismaila Soro (for the first leg against Jablonec) and Christie (on Saturday against St Mirren) that Postecoglou turned to.

Soro, Nir Bitton and McCarthy are all naturally more defensive in nature but fundamentally do not offer the same on-ball assurances that McGregor does.

It is hard to shake the feeling that not having the captain as the pivot potentially causes a ripple effect further back in terms of Postecoglou’s “ball speed, passing with purpose, the next pass always being available” mantra when his presence so clearly puts those behind him at ease.

Further forward, too, it must be reassuring for the attack-minded midfielders to know McGregor is there to feed them; he is the glue that binds the team's at-times ponderous defence with its free-flowing attack.

If someone is once again to drop out of the starting line-up for one of these games it's probably more likely to be Rogic than Turnbull and Christie is perhaps the most natural player to turn to in that case.

The Scotland international gives you the technical quality but also an added dimension of athleticism to his constant pressing and counter-pressing that could conceivably be required against better-equipped opposition but allows McGregor to continue to operate where he has for the past half-dozen matches.

Celtic Way:

What about Rangers?

Without meaning to sound glib: what about them? If Postecoglou has shown anything in his short time at the helm so far it's that he will send his team out to play the game in their own style with the belief that if they stick to the gameplan they can out-fox and out-land the opposition.

That said, there are intriguing match-ups to consider in midfield if Steven Gerrard fields a team similar to that which defeated Celtic 4-1 back in May. 

On that occasion, Rangers used an altered version of their usual 4-3-3 by starting both Alfredo Morelos and Kemar Roofe up front with Ryan Kent asked to roam in behind them. In practice that day, Kent still drifted to the left and Morelos to the right with Roofe leading the line centrally.

Celtic Way: Celtic v Rangers attacking radar from opening three league matchesCeltic v Rangers attacking radar from opening three league matches

The Ibrox side's main creative outlets may be full-backs - James Tavernier already has a league-high 13 key passes in the Premiership after three matches - but the supply line still goes through the midfield trio.

Against Celtic in May that comprised Glen Kamara, Steven Davis and Joe Aribo. So far this term those three have featured together regularly while Scott Arfield and summer signing John Lundstram are other options in there. 

Like Celtic, Rangers average well over 60 per cent possession but that dips down to less than 55 per cent in European ties – even though they were against Malmo and Alashkert.

The latter game, in particular, was notable for Rangers essentially ceding possession to Alashkert after going down to 10 men in a manner that suggested they were confident enough the Armenians would not breach their rearguard.

Given Rangers are usually perfectly content to allow teams to play around and in front of their defensive shape when they don't have the ball, the challenge Celtic will likely face is not allowing their passing game to end up exactly what McGregor's presence usually avoids: aimless passing between the centre-backs.  

When defending, Rangers coach Michael Beale has set the terms of engagement as such that one of the wingers and the central striker will usually stay high to make the opposition defence either accept a numerical disadvantage should the Ibrox side win the ball back with their pressing while their full-backs have pushed up, or force the opposition defence to stay back far enough to account for the winger and striker's presence. 

Last season Rangers scored 92 goals (75.07 xG) and conceded 13 (27.12 xG Conceded). That's a 31-goal overperformance which, frankly, will likely prove unsustainable but was sincerely impressive nonetheless.

Factor in a relatively subpar start to this campaign and the fact last season's league-winning side did give up decent chances - albeit few of them - and there will be opportunities for the likes of Kyogo to exploit. That Ross County succeeded in finding enough gaps in the Gers backline to finish Sunday's 4-2 loss in Dingwall with a higher xG than the champions reinforces that.

Celtic Way: Celtic v Rangers defensive radar from opening three league gamesCeltic v Rangers defensive radar from opening three league games

At the opposite end, it is almost inevitable that Celtic's defence will be vulnerable at some stage due to the nature of Postecoglou's high-risk, high-reward philosophy. At that stage, how well Rangers' strikers are playing and keeper Hart's influence will come into play.

AZ created enough chances to score a couple against the Hoops in the first leg but a combination of Hart's reflexes and poor finishing meant the defence left Parkhead with another clean sheet anyway. 

If that was a timely confidence-booster, just think what a successful double-header that ends in a win at Ibrox would do for morale. Those selection headaches will only get worse.