Thomas Tuchel may have to wait until after the watershed to review the video of this collapse that exposed the frailties that have been all too evident in recent weeks.
This was the first time this season they have conceded more than one goal in match, let alone three. It was also the first time they have lost to London rivals this term.
While they were let off the hook by Watford, David Moyes’ side punished them, with Jarrod Bowen the chief tormentor.
Substitute Arthur Masuaku’s stunning goal on 87 minutes sealed a miserable afternoon for the visitors and Edouard Mendy in particular, but it was the West Ham winger who had Chelsea on the rack from kick-off.
It is perhaps overstating things to suggest this result had been coming, but the cracks have been evident.
That is now two draws and a defeat in their last five league games. They have not all been like this, but fault lines have been increasingly evident.
Draws with Burnley and Manchester United were the result of careless finishing. On this occasion, it was a fragility at the back, which has reared its head in recent performances.
“It is tough to play here, we did too many individual mistakes, we did already against Manchester United and Watford and got punished,” admitted Tuchel. “If you want a result at this level you have to minimise mistakes. We talked about it before, it’s obviously not helped.
“We created a lot of half-chances, a lot of touches in the box, but sometimes you need a bit of luck, a deflected shot or loose ball that falls to our feet. Their third one is a strange one, the two big chances they had in the first half were our mistakes.
“We could not finish it with clear and better chances, be more ruthless, we struggled also in defending some situations.
“Every loss is a bump in the road, that will not hold us back from what we demand but we need to play more accurate, more stable and reduce big mistakes.”
It was Jorginho’s slack touch that gifted Jadon Sancho a goal last Sunday, and here it was his under-hit back-pass and Mendy’s indecision that handed West Ham a way back into the match after Thiago Silva had headed Chelsea in front.
Bowen was a menace all day, with Tuchel’s side failing to handle his speed or pressing. He was the man who chased down Jorginho’s back-pass, panicking Mendy into a rash challenge that prompted referee Andre Marriner to point to the spot. Manuel Lanzini converted.
This was a chance for Chelsea to move four points clear before title rivals Manchester City and Liverpool played later in the day. Instead, they provided the perfect incentive to Pep Guardiola and Jurgen Klopp with a second defeat of the season.
They opened the scoring after 29 minutes with Silva’s free header from Mount’s corner. They had come close moments earlier when Kai Havertz was unchallenged in the box, but on that occasion headed too close to Lukasz Fabianski.
Thiago had barely finished celebrating when he was called on at the other end, clearing Coufal’s shot off the line with Mendy beaten.
Chelsea only had themselves to blame when West Ham did find an equaliser on 40 minutes, with Jorginho and Mendy both culpable in conceding the penalty which Lanzini converted.
First, Jorginho’s under-hit back-pass gave Bowen something to chase. But Mendy still had two opportunities to clear the danger before bringing down Bowen under pressure.
Lanzini stepped up and fired high into the top left-hand corner, sending the Chelsea goalkeeper the wrong way.
Tuchel was visibly frustrated on the touchline, with this the latest occasion when his side have created their own problems at the back. But Chelsea regained the lead through a goal of real quality from Mount just before the break.
Hakim Ziyech’s cross was lifted to the midfielder, who was racing into the box on the right-hand side. He had time to watch the ball all the way and beautifully disguised his shot to beat Fabianski at the near post with a side-footed volley.
An injury to Havertz in stoppage time meant Tuchel had to make a change at the break – bringing on Romelu Lukaku, but he also had time to reorganise his side, who should have been better prepared to handle West Ham in the second half.
Instead they were level again 11 minutes after the restart, with Bowen proving too much for them to handle.
Michail Antonio’s clipped ball forward was half-dealt with by Silva, but Alonso was too weak under pressure from Coufal, who played in Bowen. The winger’s powerful, low drive found the bottom corner of the net and left Mendy sprawling.
The game still felt like it could have gone either way, with West Ham threatening when bursting forward, but Chelsea failing to create enough meaningful chances.
Even still the winner came totally out of the blue with Masuaku’s apparent cross swerving suddenly and beating Mendy at his near post.
Replays did not make it clear whether the effort took a deflection off Ruben Loftus-Cheek, but Mendy’s positioning was still questionable.
The biggest questions right now, however, are over Chelsea’s title credentials.