How 2016-17 Expected Goal and Save Values Help Illustrate What’s to Come in the Premier League
Either Pep Guardiola has been taking note of some new metrics or his football sense is just that keen. Regardless, the argument can be made that Joe Hart had the last laugh.
For last season, that is.
The goalkeepers Guardiola enlisted over Hart didn’t only fail the eye test as Manchester City fell 15 points shy of the Premier League title in the manager’s first season. Claudio Bravo and Willy Caballero were a measurable problem for Guardiola, and the manager wasted little time rectifying that this transfer window by bringing in Ederson from Benfica.
STATS has refined some of the most advanced metrics in football, and using them gives insight into just how teams measure up against expectations. Expected goal value uses machine learning and historical tracking data to address how likely a goal is based on the location of a shot, the position of the defenders and manner of the attack. Possibly the best way to think of it in terms of how it measures a player’s worth is that it assesses the individual against the league average.
Man City allowed 39 actual goals. Their expected goals against came in at 36.9, and the plus-2.1 differential – meaning they allowed more goals than they should have – accounted for the fourth worst in the league ahead of only Crystal Palace, Watford and Liverpool. The contributions of Bravo and Caballero with expected save differential – calculated by subtracting expected saves from actual saves – was minus-5.7, meaning they did not save nearly six more shots than the average keeping tandem would have. That ranked ahead of only Crystal Palace’s keepers. With Hart featuring the year before Guardiola’s arrival, City were at a perfectly acceptable shade above the league average at +0.1.
That very last line of defense is, of course, not the only area where City spent this summer. The signings of Benjamin Mendy, Kyle Walker and Danilo figure to address any defensive shortcomings for a side that’s dealt with inconsistency and injury even after bringing in John Stones last year. Without even getting into the addition of Bernardo Silva and having Gabriel Jesus for an entire season, those signings may amount to the changes City need to bring Guardiola yet another trophy in yet another league.
If you’re not yet convinced, let’s now consider improvements they could see from within their established attack by surveying Guardiola’s most trusted finisher and his supporting cast.
Sergio Aguero’s -4.1 expected goal differential – calculated by subtracting an individual’s expected goals from converted goals – last season was the third worst in the division, but he was at +3.6 the previous season. He still scored 20 goals last season, and he did so while wasting chances. Of the seven 20-goal scorers from the past two seasons, he’s the only to post a negative xG differential. That’s a class he’s repeatedly been a part of, so it probably follows that he’s quite unlikely to waste as many chances going forward.
Now consider David Silva (-1.9), Raheem Sterling (-1.8) and Kevin De Bruyne (-1.6). Silva was right at that rate in 2015-16 while Sterling was slightly better (-1.0), but De Bruyne’s was +2.5.
If Aguero and De Bruyne get back up to a level we know they’re capable of and City put them in similar situations to score, the tandem could theoretically account for 11 more goals. Add that to the prospective goalkeeping improvement, and City have the possibility for a staggering overall goal differential increase.
But a similar argument can be applied to the attack of City’s closest rivals, who spent big this summer on efficiency they sorely need. If that works out, the managerial rivalry we saw between Guardiola and Jose Mourinho elsewhere could reach maximum velocity in England.
Manchester United’s potential for improved efficiency is enormous
After a 4-0 loss at Chelsea on Oct. 23, Manchester United found themselves five points back of the eventual Premier League champions.
From there, the Red Devils allowed 17 goals in 29 league matches, conceded more than a goal once and kept 14 clean sheets. On paper, that certainly looks like the defensive capacity to give a club every opportunity to make up those points to win the league.
Yet they drew 13 of those 29 matches, lost two and did not traditionally qualify for the Champions League because of it. Their five goalless finals in that time included Old Trafford disappointments with Burnley, Hull City and West Bromwich Albion.
To say Mourinho’s side left points on the pitch is an understatement, but how to quantify it? United’s goals for differential was substantial. Try -16.3, or third worst in the division behind Southampton (-27.6) and Stoke City (-17.2).
Remember, that’s how we quantify finishing measured against the league average. It was a systematic problem of wasting chances with no player posting an xG differential of even +1.0. Jesse Lingard (-3.4), Paul Pogba (-3.3), Marouane Fellaini (-2.8), Zlatan Ibrahimovic (-2.4) and Wayne Rooney (-2.0) were the most responsible parties. Some of those names are gone, but attackers like Marcus Rashford and Antonio Valencia also produced negative differentials.
In this case there seems to be a need to address styles of play. If Mourinho is able to do that and get even average outputs from key attacking players, the shift in goals scored could be significant enough to make United – not City – the team to bring Manchester back to the top.
Now add a new No. 9 to the mix. United’s potential appeal for 2017-18 only increases with Mourinho’s addition of a player who could sway efficiency back in the right direction – if United use him properly. Romelu Lukaku’s +9.9 xG differential might not be sustainable on quite that level, but there isn’t much of an argument against Mourinho having added a finisher who operates at impressive efficiency levels to an attack that’s already bursting with measurable potential.
Lukaku’s xG differential trailed only Harry Kane last season. That begs this question: If Spurs couldn’t win the league last year, is it realistic to think they can now?
Last season might have been Tottenham’s best chance
Tottenham had some of the most quantifiably effective attackers in the Premier League last season, and it goes well beyond Kane.
Kane scored 29 goals in 30 matches, which is impressive enough on its own, even if you normalize the seven goals he scored in two throwaway matches at season’s end. It becomes even more remarkable when considering his 15.7 xG. His league-leading +13.3 xG differential implies he was consistently finishing chances the average player wouldn’t.
He also led the league in 2015-16, but with a considerably lower and more sustainable differential (+5.2). His mark last season was significantly better than next-best Lukaku, and, for comparison’s sake, any of the big names for the Spanish giants. Lionel Messi led in Spain with a +9.3 xG differential, and no one in the Bundesliga, Ligue 1 or Serie A topped that.
It wasn’t just Kane for Tottenham, though it was an alarmingly top-heavy club performance. Spurs scored a league-best 86 goals, which was +17.6 of their expected goals for. But their xGF differential was thanks entirely to three of the top six individuals in expected goal differential being a part of White Hart Lane’s final season. Heung-Min Son (+6.2) ranked fourth and Dele Alli (+5.3) was sixth.
The three scored 61 of the club’s goals despite Kane missing eight matches. Spurs lost none of them and dropped six points, which still wouldn’t have been enough to win the league. The success without Kane hinged heavily on eight combined goals from Son and Alli.
Tottenham’s depth seems questionable and is already under examination with Walker joining City and replacement right back Kieran Trippier suffering a preseason ankle injury that will cost him the start of the season. How Mauricio Pochettino closes the gap between Tottenham and the top with a fully healthy and optimally efficient side is no small question. Consider any personnel concerns that may arise, or any dip in form, and producing the level of efficiency they’d need to win the league seems nearly impossible.
Hugo Lloris’ stellar 2016-17 furthers that point. Tottenham’s expected goals against was 36.0. They conceded just 26, for an xGA differential of -10.0. Lloris and Michel Vorm combined to post the league’s second-best expected save differential with a +10.4 mark, meaning they saved at least 10 goals a league-average keeper would have let by.
For that to be sustainable, Lloris will have to prove Tottenham’s consistent xS differentials of years past – +0.7 in 2015-16, +0.2 in 2014-15 and +1.1 in 2013-14 – were somehow the anomaly rather than the norm.
That’s a glimpse into how Spurs could struggle to entertain Wembley. But even their North London rivals who perform better at those grounds could have similar concerns.
Cech can’t save Arsenal to the top
Spurs’ goalkeeping was great. Arsenal’s was slightly greater, but that has to be of the greatest concern to Arsene Wenger for reasons beyond his No. 1 being on the wrong side of his prime.
Petr Cech helped the Gunners to a league-best +11.7 xS differential, which was the best in the Premier League over the past five seasons.
It’s difficult to see this as sustainable, particularly since Arsenal’s xS differential in Cech’s first season at the club was +4.4. While it should be of some comfort that they added Ligue 1’s most efficient striker in Alexandre Lacazette (+8.3 xG differential) to balance that on the other end, there are further reasons for concern.
Arsenal gave up 44 goals as it was. If we add seven to that, bringing them in line with their 2015-16 save differential, their goals against last season jump to a tie with West Bromwich Albion for eighth at 51. No club in the last 15 Premier League seasons has conceded more than 50 goals and finished in the table’s top four. Put the Gunners at the league average by adding 11 goals to their save differential, and they’re tied with Burnley for 10th in goals against.
And if winning the league is their ultimate goal, Arsenal have a great deal of work to do with a back line and midfield not so dissimilar to last season’s. No club has ever won the Premier League while allowing more than 45 goals.
Maybe that’s where Chelsea finally come in as the London club with the best shot at the title.
A third title in four years?
The Blues conceded 33 goals last season, which was third to Manchester United and Tottenham. That number matters because it was consistent with the club’s expected goals against (31.8). They didn’t have a keeper constantly bailing them out. Their system worked.
The problem for Chelsea comes with scoring, where Antonio Conte got much more last season than his club may be capable of moving forward. Their 85 goals for ranked second to Tottenham, but their +20.6 xGF differential was a full three goals ahead of that previously discussed unsustainable Kane-Alli-Son-powered mark for Spurs. Granted, Chelsea’s productivity was more spread out among attacking weapons, their stability absolutely thrived in a three-back system, and they added Alvaro Morata’s potentially impressive efficiency.
Those three terms – productivity, stability, efficiency – are telling, and they’re more measurable in football than ever. Conte comfortably won a title in his first season at Stamford Bridge by implementing them in impressively quick order.
But that could ultimately mean little this season considering Guardiola’s established plenty of his own.