There are no easy explanations for Chelsea’s problems. The Blues have fallen from deserving title winners to 15th place in the span of one summer. Jose Mourinho is still managing the same players—there would be no exodus of stars for a Roman Abramovich owned superclub. They have not been particularly unlucky, either. By expected goals, Chelsea should have goal difference of roughly zero. That is slightly better than the club’s real GD of minus-4, but hardly good enough to compete for top four, let alone the title.

The following graph shows chances created and conceded by Chelsea, with the size of the marker representing the expected goals value of the chance. Pink boxes are goals.

Opposition clubs have finished well against the Blues, but the overall picture shows Chelsea failing to assert control over matches.

This demands explanation. The most comprehensive attempt to lay out what has gone wrong was written by Sebastien Chapuis. In this piece, I want to add one important piece to the puzzle. Chapuis mentions that Chelsea had depended on its first XI quite heavily during the 2014-15 season. This can be quantified, and the results are striking. In this analysis I removed goalkeepers, as they do not suffer from fatigue in the same way as outfield players.

In 2014-15, Jose Mourinho depended on his top regulars more than any other recent Champions League club in the EPL, and by a significant margin. I compared how many minutes each team’s regulars played to the total possible minutes they could play between the EPL and the Champions League.

The six top Chelsea regulars (Branislav Ivanovic, Eden Hazard, John Terry, Nemanja Matic, Cesc Fabregas and Gary Cahill) played about 92 percent of all possible minutes in the EPL and Champions League. The top ten (which adds in Willian, Cesar Azpilicueta, Diego Costa and Oscar) covered about 81 percent of possible minutes. Since the 2010-2011 season, no other team had depended on their top six regulars for 85 percent of possible minutes, or for 75 percent of possible minutes from the top 10 regulars.

The same is notable, perhaps even more so, with the use of substitutes. Most Champions League clubs expect their six top substitutes to cover about 35-40 percent of possible minutes. Chelsea asked for just 25 percent of possible minutes from these subs. Mourinho kept his subs on the bench and his regulars on the pitch.

This almost certainly made Chelsea better in the 2014-15 season. The best players spent more time on the pitch. Maintaining the same XI helps these players develop understanding, as Daniel Altman has suggested. But this may not be a sustainable strategy. Given that no one else in recent seasons has tried to squeeze so many minutes out of so few players, it seems reasonable to guess this was a risky strategy and that a serious hangover the next season was one of those risks.

It is particularly striking that among the hardest-worked players in 2014-15 are four players — Eden Hazard, Nemanja Matic, Cesc Fabregas and Branislav Ivanovic — who have been among the most disappointing performers in the current season. Coming off 4000-plus minute seasons, they may have needed extra recovery time to return to form. Further, Chelsea’s front office failed to acquire new frontline starters to cover minutes that key contributors like Hazard, Matic, Fabregas and Ivanovic might not have been ready to manage. So instead Chelsea have continued to count on guys who seem worn out and who have consistently failed to play at the level the club needs.

On Saturday, Chelsea get another shot at righting the ship with a home match against Liverpool. If one of the club’s problem is this kind of generalized fatigue, it is unknowable when exactly the players will snap out of it. It could be tomorrow. But I do think a significant chunk of Chelsea’s early struggles can be explained by fatigue and overuse from last season. Exactly how much this will cost them is still to be determined.