Tinker, Tailor, Mould a Side: Squad Rotation in the EPL



One of the key features of Chelsea’s title winning season was the consistency of their starting lineups. After the 3-0 thrashing by Arsenal in September, Conte switched to a 3-4-3 formation and Chelsea embarked on a 13-match winning streak in the league that ultimately propelled them to the title. The foundation of this formation – Luiz, Cahill and Azpilicueta – started each of the next 32 games, and the wing-backs, Moses and Alonso, missed only three between them.

Such consistency is partly due to luck with injuries and suspensions, but Conte also resisted the temptation to tinker with his team. Other managers opted to ring the changes, for tactical purposes or to rest players. In the closing weeks of the season Mourinho was compelled to defend his rotation policy, citing fixture congestion and the need to maximize his chances of Europa League success. However, frequent changes to United’s starting lineup were a feature of their entire season not just the final few months.

In this article I’m going to take a detailed look at how EPL clubs utilized their squads throughout the season. I’ll compare the rate at which managers ‘rotated’ their teams (which I define simply as the number of changes made to their starting lineup) and the number of players they used in doing so. I’ll investigate some of the factors that may have influenced a manager’s decision to fiddle with his lineup. Finally I’ll discuss whether rotation had an impact on results.

Squad Rotation


Let’s start with a look at squad size and rotation. Figure 1 plots the average number of changes made to the starting lineup against the total number of players used by each EPL club last season.

Clubs on the left-hand side of the plot preferred to maintain the same starting lineup, changing about one player per match. Those plotted towards the right of the plot varied their team more frequently. The vertical axis measures effective squad size – the number of players that started at least one EPL match[1]. Teams that are plotted towards the bottom of the plot picked their lineups from a relatively small group of players, those plotted nearer the top chose them from a larger pool.

Figure 1: Squad rotation (average number of changes made to the starting lineup) versus effective squad size (number of players that started at least one league match) for all EPL clubs in 2016/17. Uses data provided by Stratagem Technologies.

Both quantities plotted in Figure 1 are important. A manager could adopt a highly structured rotation policy in which three players are changed in each match but are chosen from a small squad of only 14 players; this club would appear in the bottom right of the plot. A manager that was struggling to find his best eleven might make the same number of changes per match but from a much larger pool of players; this club would appear near the top right of the plot.

On average, EPL clubs made around two changes per match to their starting lineups, from an effective squad size of twenty-five players. As you might expect, there is clearly a relationship between squad size and rotation: the more frequently that a club rotated, the greater the number of players they tended to use. West Brom, Chelsea, Burnley and Liverpool, who made just over one change per game, fielded the most consistent lineups. Along with Spurs, they also used the fewest numbers of players[2].

At the other end of the scale there is the two Manchester Clubs – both of whom made over three changes per game to their starting lineup – followed by Southampton, Middlesbrough, Swansea and Sunderland. Man Utd and Sunderland, along with West Ham and Hull, all used at least 28 players over the season (admittedly United started 5 players for the first time in their last game of the season).

So there was quite a broad spectrum of squad management styles in the EPL this season, with some clubs rotating more than twice as fast as others and using nearly 50% more players. Why is this? To what degree are team changes enforced or by choice? I’ll now review some of the factors that may have influenced team selection.

Injuries


Injuries and suspensions will have forced managers to make changes to the team. According to physioroom.com, Sunderland suffered the most injuries of all EPL clubs last season, 81 in total[3], with Man United, West Ham and Watford all receiving over 70. Chelsea, West Brom and Burnley were much luckier, suffering about half as many. Liverpool were the black sheep: the only other team to suffer over 70 injuries, but still one of the most consistent starting lineups. I haven’t found data listing the total number of players suspended at each club last season, but Man City, Sunderland, Hull, West Ham and Watford all received at least four red cards, whereas Liverpool, Chelsea, Spurs, Palace and West Brom received none.

In general, there is a weak correlation between both Transfermarkt’s ‘fair play’ score and physioroom’s injury counts and the squad rotation metric used in Figure 1. This suggests that while injuries and suspensions will have contributed to squad rotation, they were not the main driver.

Fixture Volume


Fixture volume over all competitions seems to have influenced some club’s selection decisions. EPL teams played an average of 47 matches this season (which is exactly the number that Chelsea played); Man United played 64 – over a third more – and Man City 56. Generally speaking, teams that played more than 50 matches tended to rotate their league teams more frequently than those that played less, although Sunderland and Middlesbrough both played only 43 matches. European competition is one of the biggest sources of additional matches; in a previous blog I’ve demonstrated that playing in Europe does affect domestic results in the EPL.

A Settled Defence


A key feature of the clubs that rotated the least was a settled defence. Throughout the season Burnley and Chelsea fielded only 6 unique combinations of players in defence, and their preferred defence started more than 25 league matches. In contrast, most EPL teams tried more than 15 different combinations of players in defence, with their most frequent combination typically starting around 12 matches. The teams that rotated the most – those towards right of Figure 1 – never really established a first-choice defence.

The following plots emphasize this point. They show the starting lineups of Burnley and Man City in each of their 38 league matches last season. A dot indicates that a player started the match, with blue dots indicating players that were retained in the starting lineup and red dots showing those that were brought into the starting team. The result of each game is given along the bottom. Similar graphics for all EPL teams can be found here.

The difference between the selections in defense is striking. Both clubs used seven defenders over the course of the season. However, while Burnley’s first-choice back four is obvious, City’s certainly is not. Over the course of the season they tried 21 different combinations of players in defence, which is well over half the total number of unique ways you can select 4 players from 7[4]. City’s most frequent combination in defence was Kolarov, Otamendi, Sagna and Clichy; they started a grand total of 4 matches together.



Indecision


It took some managers several matches at the start of the season to identify the core players around which their team could be constructed. Others took much longer decide on their strongest lineup, and a few never did.

For example, David Moyes never really figured out his best team as the plot below demonstrates. He deployed 36 unique combinations of players during the season (nearly double that of Chelsea), and there was a lack of consistency in defence and midfield, both in terms of personnel and formation. Jose Mourinho also tried a large number of different combinations in every position, particularly in the second half of the season. While United’s rotation frequency certainly increased in the last couple of months of the season, they were already rotating at over 3 players per game before Christmas.



Does rotation matter?


Is there any evidence that rapid squad rotation influences results? This is a tricky question to answer because we don’t know how a team would have performed had they been more or less consistent in their team selection. Periods of high rotation do seem to have coincided with worse results for many teams (West Ham, Watford, Crystal Palace and Swansea, to name a few). However, there is a bit of a chicken-and-egg issue: poor results may compel a manager to change his team until he finds a winning formula.

I find there to be no significant relationship between squad rotation and final league position last season. However I would hazard the suggestion that the majority of teams that prioritized stability and a tight-knit squad – those nearest the bottom left corner of Figure 1 – all had successful seasons by their own standards. Crystal Palace are perhaps the exception, but the rate at which they varied their starting lineup dropped significantly (from two changes per game to one) in the second half of the season once Sam Allardyce took charge.

Similarly, those clubs that rotated frequently from a big squad generally had a disappointing year relative to pre-season expectations: City failed to mount a sustained title challenge, United finished sixth, and Hull, Swansea, West Ham, Middlesbrough and Sunderland were either relegated or flirted with relegation.

Perhaps this is just postdiction, but I think it warrants further investigation. It would be interesting to establish whether the performance of a team tends to decline towards the end of a long season if players are not rested. Are big squads problematic if managers are forced to rotate simply to keep his players happy? Does rotation interrupt momentum?

No Europe and a lack of injuries have helped, but the last two EPL seasons have been won by clubs that identified their best 11 players and stuck with them; tailoring and not tinkering. As clubs recruit over the summer we’ll see whether this is a theme that has started to resonate.

Thanks to David Shaw for useful comments. Lineup graphics for all EPL teams can be found here.

This article was written with the aid of StrataData, which is property of Stratagem Technologies. StrataData powers the StrataBet Sports Trading Platform, in addition to StrataTips.


[1] I’ve investigated other measures of squad size, in particular counting the number of players that started at least 2 or 3 games; none of the conclusions would change significantly.

[2] Note also that Chelsea used three players for the first time in their second-to-last game, when they had already won the league

[3] Injuries are counted from the first weekend of the season. It’s worth on noting that this count includes injuries to squad players who weren’t necessarily in the starting lineup.

[4] Furthermore, City also experimented with Fernandinho or Navas as right full backs, so arguably their defence came from an even bigger pool, and was even more erratically selected.

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