Ange Postecoglou faces Celtic full-back puzzle after Josip Juranovic debut - James Dailey

With the recent acquisition of Croatia international Josip Juranovic and the ascent of Anthony Ralston, which full-backs are likely to be first-choice is up for debate.

Juranovic started in place of Greg Taylor at left-back against Rangers on Sunday and performed admirably on what was his Celtic debut. In fact, according to WyScout, he had played just 508 out of 17,836 minutes at the senior level on the left side since the start of the 2015-2016 season.

Assuming Juranovic has been brought in to be the first-choice full-back, is he more likely to displace Ralston or Taylor? Of course, rotation may result in a mixture, but my column today is dedicated to examining the question of who will play where, and what the associated positives and negatives may be.

First, let us take a look at some historical metrics on Taylor and Ralston. I have compared each of their age-21 seasons in the SPFL Premiership.

I conducted this comparison months ago and shared it on Twitter at the time, as my premise had been that sentiment towards Ralston appeared overly negative. While his creative output, crossing and passing metrics were lower than Taylor's, he was more active defensively and suggested some athletic upside via his superior dribbling and progressive run metrics.

While the data are limited so far this season in league games, they generally fit with their age-21 season profiles:

Generally speaking, Ralston has played less advanced than Taylor on average and been less involved in attack. However, his creative output has been good, if not quite up to Taylor’s level.

Defensively, Ralston has been more aggressive and better at defending Celtic’s box, aided by superior aerial presence relative to Taylor.

Taylor has been a bit better in counter-pressing, though that may be partly as a result of his more advanced average positioning.

Why would it even make sense to consider playing Juranovic out of position as a left-back? Maybe it does not overall, as his attacking profile as a right-back has been excellent so far in his career. However, Juranovic is not a tall man for his position, and his historical success rate in aerial duels is unsurprisingly low at just below 45%.

But why would I care about aerial performance levels of Celtic full-backs?

READ MORE: Is Lewis Ferguson Celtic's long-term answer in central midfield?

Beyond the obvious issue with regards to Celtic’s struggles defending set-pieces over the past couple of seasons, centre-backs Stephen Welsh and Carl Starfelt have not been particularly strong in the air either.

Again, while limited in sample size so far this season, their metrics largely conform with their careers to date:

One of the tactics AZ Alkmaar and Rangers appeared to deploy was playing long diagonal passes trying to get behind Celtic’s full-backs, but also trying to attack Celtic’s centre-back pairing with the potential upside of winning second balls.

Additionally, the recruitment and installation of Joe Hart as Celtic’s number one keeper may result in better shot-stopping over time, but he has to date played the position deeper than Ange Postecoglou’s system typically requires.

Here is Hart’s heatmap from Sunday’s game against Rangers, then Scott Bain’s versus Midtjylland, and then Yohei Takaoka’s for Yokohama F. Marinos playing in Postecoglou's system in their most recent game versus Kashima Antlers:

Joe Hart heatmap v Rangers

Scott Bain heatmap v FC Midtjylland

Yohei Takaoka heatmap v Kashima Antlers

We could see the significant presence of Bain and Takaoka out in the 'sweeper-keeper' role approaching the centre circle, while Hart has tended to stay closer to his penalty area.

Now I’ll draw your attention to the Average Defensive Action Distance metric in the centre-back radar for Starfelt and Welsh, which is a proxy for how high of a defensive line they have played. We saw these factors converge against AZ Alkmaar for their first goal last Thursday, as Welsh’s awkward aerial deflection of the ball was played back to Hart, who was positioned at the top of his penalty area.

That space between Hart and Celtic’s centre-backs may be a structural vulnerability while Hart plays in Postecoglou's system. Given he is somewhat uncertain on the ball and has relative immobility compared to a traditional sweeper-keeper, it may be wise to have him deeper.

However, a full-back tandem of Taylor and Juranovic is unlikely to improve this vulnerability to aerial attacks. In fact, it may further incentivize opponents, which in a domestic league full of more aerially-focused teams, could be an issue.

For example, last season the league average for aerial duels per 90 minutes was about 56. Compare this to the Dutch Eredivisie (33), English Premier League (34) or even English League One, which averaged fewer than the SPFL Premiership at about 54.

Would playing Juranovic on the left be ideal? Probably not. Perhaps Liam Scales will offer some greater size in that position as he settles and if he is up to playing that position at a new level.

Regardless of which iteration Postecoglou decides, Celtic are likely to continue being challenged in the air.