Head Bhoy

Kristoffer Vassbakk Ajer (to given him his full Sunday name) has enjoyed a meteoric rise as he passes his 20^th birthday.

From captaining IK Start in Norway’s Tippelgaen at 16 years old, he signed for Celtic at 18. Unusually, he made a success of a loan move to a team in the same division (Kilmarnock). 17 appearances followed a January 2017 loan deal with positive reviews. Back on the bench for the start of the 2017/18 season at Celtic, he nevertheless is currently a first team regular. He has secured his first major honour – the Scottish league Cup. A remarkable rise was capped (literally), twice, with appearances for the Norwegian full national team during the most recent international break.

Despite his youthfulness, he has already appeared in 138 matches including Norwegian international age group football. Even more remarkably, he has achieved all this whilst adapting to a new country and a new position. Primarily a midfielder at IK Start, he has been transformed into a central defender by Celtic. At 6ft 5in this appears lazy stereotyping. But despite his size he is primarily a player comfortable with the ball at feet. In other words, he fits the modern template.

With Simunovic on the bench, Boyata remaining skittish, Hendry finding his feet, Sviatchenko with one foot out the door on loan back in Denmark and Compper having disappeared, Celtic are left with Ajer as the one central defensive constant as the business end of the season looms. He has started 21 out of the last 22 games. His run began after the 4-0 debacle at Hearts.

Even more surprisingly, the junior partner has been chosen as the left of a two fifteen times – he is right footed. He has four times been the central of a three and twice the right centre back. He even started at right back against The Rangers at Ibrox. But Celtic have only conceded 13 goals in the 21 matches he has started.

As I move into the analysis, remember that The Glossary is the big lad at the back who likes a tackle.

Note: player analysis data does not include the recent 2-1 loss at Hibernian.

Defensive Analysis

DASR

A reminder that defensive indicators are not as advanced as attacking measures such as Expected Assists and Expected Goals. Individual attacking actions are easier to analyses compared to defending which is inherently a collective discipline. But we do try!

Caveats aside and getting to the core of defensive performance then. Successful Challenges and Interceptions measure success in all challenges whether aerial or ground based plus attempts to intercept the ball.

In terms of Celtic defenders with over 900 minutes this season (all values are per 90m):

Bear in mind this will be higher for central defenders over full backs. This is the bread and butter of defending and Ajer wins significantly more challenges and intercepts than his peers. Broken down he averages:

• 5.85 Aerial challenges won per 90m
• 3.09 Tackles won per 90m
• 2.87 Interceptions per 90m

Boyata wins 4.30 aerial challenges per 90m and would be the next highest. Ajer is clearly very tall but that doesn’t necessarily mean good aerial technique. However, he is dominant in the air, losing only 0.94 challenges per 90m. His most memorable game in this regard was at home to Aberdeen in December when the Dons insisted on punting a steady stream of high balls in his direction. He won 13 aerial challenges, losing 2.

This is one side of the Defensive Action Success Rate (DASR) metric, the other being Unsuccessful Challenges and Fouls Conceded.

Again, only including those with > 900 minutes (all values per 90m):

Simunovic leads this side of the equation. You will see from the Successful Challenges and Intercepts he commits to less challenges overall. I like that in a defender – the old maxim about tackling being a last resort. Still, not generally how Scottish football is played.

Again the full backs don’t come out well on this but we can probably excuse Tierney based on how little defending he actually does.

When you put these two metrics together as a proportion you get DASR, a single metric of defensive effectiveness.

When I have compared all centre backs using DASR, it is noticeable that the range is between low 70%s and 81%. The gold standard is Van Dijk (I only have one season of data for him). His DASR in that last season at Celtic? 81%.

So the margins might seem small but 81% is the highest DASR score I have recorded and no other centre back reaches 80% in the last four seasons.

Ajer’s score is therefore impressive.

Bitton probably does better than many would expect but there are other variables to consider in defensive performance as we’ll see. Over the last four seasons most Celtic defenders are around the mid 70%s. This seems to be the benchmark of average. We want 80%!

Possession Won and Lost

Put simply, you want your defenders to win back possession often, and lose it hardly at all. Comparing Possessions Won to Possessions Lost per 90m gives us this:

Boyata and Simunovic have the lowest possessions lost rates but overall Ajer has the highest differential between possession won and lost. Bitton wins a lot of possession but loses it twice as much as Boyata and Simunovic highlighting again he has great strengths in this position but also glaring weaknesses.

Errors

Error measurement is subjective on my part but I am looking at positioning, marking responsibility, misplaced passes in dangerous positions, miss controls. Centre backs error rates are particularly key as the consequence of any error is likely to be severe given their central location on the pitch and being the last line before the goalkeeper.

As a benchmark, Ambrose averaged over 1 defensive error per 90m, and Boyata was at 0.95 in his first season, since reduced greatly. Around 0.5 per 90m seems to be “normal”.

This season I have been recording the Expected Goals resulting from errors. This provides a potential impact from the errors. For the squad defenders, here are current Defensive Errors and the xG resulting (both per 90m):

As discussed earlier in the season, the Bitton at centre back experiment was not something I was a supporter of. It is a great idea against average SPFL opposition where he has time on the ball to start attacks from deep with his good range of passing. Against better opposition and especially in Europe, his concentration and positioning was found out time and again. But as you can see from the table above, although Bitton’s error rate is pushing up to Ambrose proportions (0.83 per 90m), the potential impact of those errors was relatively low. 0.054 xG per 90m is equivalent to a shot from outside the box.

Conversely, Simunovic’s error rate is around the 0.5 average I mentioned (0.54), but the potential impact of his errors is much higher (0.077 per 90m). You could argue he has been unlucky, or Bitton lucky. Boyata’s error rate has come down a lot since his early Celtic career, although some in the support are resistant to acknowledging this.

But what of our subject, Ajer? Of the centre halves he has the lowest error rate and lowest xG resulting from those errors.

What is especially interesting is that Ajer’s errors have tended to come against the less stellar opposition. Two against Partick Thistle away and two and Hamilton Academical away being his most error prone performances. Meanwhile appearances against Rosenborg, Astana, Aberdeen, Zenit St Petersburg, The Rangers and Hibernian have been mainly error free.

Out From The Back

Celtic’s preferred style is to play out from the back and maintain possession. The centre halves must be comfortable on the ball and be able to feed the midfield.

Passing data for the defenders:

Ajer averages just under 70 completed passes per 90m with 96% accuracy which is what you’d expect from a centre half i.e. low risk passing. More interestingly he is bottom of the defenders league table in Passing Impect. This an indicator of the number of opponents taken out the game with forward passes. I expected Ajer to be higher given his history as a central midfielder. Bitton, also a converted central midfielder, scores highest here and Ajer’s per 90m average score is less than half that of the Israeli.

Ajer also creates less chances (0.23 per 90) than all but Simunovic and has the joint lowest Expected Assists (0.018 per 90m).

Ajer may be playing it safe with his passing given his youth and inexperience of the position – he will not be looking to overcomplicate his game.

That being said, as analysed at length in this excellent article The Dribbler: A Game-Changing Talent from 90 Minute Cynic, Ajer has created his own signature move – the dribble out from the back. A player of his size hurtling up the field is quite a sight. And highly effective in disorganising opposition defensive lines.

But he doesn’t do this as often as you may think. Here are the top dibblers with successfully completed dribbles per 90m and their Imepct score;

Ajer only completes on average just over 1 per 90m (but slightly more than Sinclair – eek!). He is not in the same league as the primary wingers nor the rampaging Tierney. And Bitton is not far behind him in this regard. We probably notice Ajer’s sojourns because of their effectiveness – his average Dribble Impect is significantly higher than Bitton.

Something we’d like to see more of.

Goal Threat

Ajer scored 14 goals from midfield in Norwegian football, averaging 0.268 goals per 90m. Which is quite something for a teenager in senior football. In addition he has racked up 10 goals in 39 appearances for the Norwegian International age grade sides. He has yet to score for Celtic though.

Of all the regular defenders he has the 2^nd lowest Expected Goals per 90m at 0.046. His shots within the box and overall possessions in the box are broadly similar to the other central defenders.

Summary

Ajer’s career trajectory is firmly on the upward path at the moment. He has settled into the Celtic first team and kept his place with extremely good defensive statistics.

That is not to say there are not areas requiring improvement. He tends to grapple opponents very noticeably with his hands, and has been pinged for that on a few occasions. He shares a fault common in all the Celtic centre backs of getting too tight to opponents on occasion. This can result in losing a flick on or worse, being turned. He needs to learn when to drop off, when the ball cannot be won without fouling, and defending from there i.e. stopping the turn or the next action.

There are also the odd rash moments such as in the Scottish Cup semi-final when he was pulled out wide and dived in on Candeias leading to a moment of danger.

Ajer is currently playing a risk averse game apart from the glorious up field forays with the ball at feet. Given his midfield background he can improve his range of passing. Similarly, he is due a goal or two!

But overall his core job is defending and with Van Dijk-esque numbers posted so far, the kids doing all right.