Thomas Tuchel was left to rue a catalogue of "big mistakes" after Chelsea slumped to their second Premier League defeat of the season at West Ham.
The Premier League leaders travelled across the capital knowing victory at the London Stadium would move them four points clear of title rivals Manchester City and Liverpool before they even had the chance to kick a ball this weekend.
But Chelsea paid the price for an error-strewn performance as goalkeeper Edouard Mendy was at fault for two of West Ham's three goals, including Arthur Masuaku's freakish winner.
"I didn't see our performance dip against Manchester United or today, it was enough to win the game," Tuchel said.
"We gave easy goals away, this is unusual - big mistakes that lead to big chances and easy goals. We've done this against United, Watford, and today.
"We are not good enough in the details at the moment. This is a game about details, I can only repeat myself. Without doing big mistakes, there's a big chance we win this game.
"The [title] race is not interesting at the moment, what is interesting is the next 90 minutes. We have to have the focus right, the attitude right, and to take care of the details because we are not good enough with the details at the moment.
"If you want to protect yourself against counter-attacks, it is impossible to do these kind of mistakes. We have done these three times in a row; this is simply too much."
Thiago Silva's towering header had given Chelsea the lead at the London Stadium only for Mendy to bring down Jarrod Bowen in the area after controlling Jorginho's backpass, with Manuel Lanzini converting from the penalty spot.
Mason Mount's stunning volley restored Chelsea's lead before the break but it would count for nothing as Bowen levelled for the second time early in the second half before Masuaku's speculative cross somehow found a way past Mendy to seal the West Ham victory.
"Edouard is going through a bit of a rough period, I don't know why exactly, but he seems to have lost a bit of confidence," Tuchel added. "Decision-making was not of the highest level with the first goal but we also brought him into a really awkward situation.
"He could have saved the situation, he did not. You can turn it and twist it, but this is far, far away from blaming anybody. We need to admit that we are doing too many crucial mistakes at the moment that cost us a potential win against Manchester United and another today, potentially."
In December 2020, a month before Tuchel was appointed successor to Frank Lampard, Chelsea won just two of their six Premier League games. With Manchester City and Liverpool lurking over their shoulders, this season's title challenge can ill afford a repeat of such form.
Asked if he had considered his players may be suffering from heading into the festive period, Tuchel responded: "I cannot give the excuse that we are tired, it's absolutely normal to be tired if you play Premier League through winter.
"It is a question of absolute focus, it's a question of detail, it's a question of precision and risk management - where do you take the risk and where you absolutely don't take the risk.
"If you invite a team like west ham into your own box or the final third by big ball losses then you are in trouble."
Moyes: Hammers can play much better
Victory over Chelsea is always welcome for West Ham, but this was all the sweeter after three matches without a win and cements fourth place in the table with a four-point buffer now between the Hammers and Arsenal in fifth.
David Moyes' side had already beaten Liverpool at home this season and can now add the European champions to their list of high-profile scalps.
"I think we played better against Liverpool than we did today," Moyes said. "I think we can play much better than we did. We didn't play well enough in the first half, in the second we improved greatly and that gave us a real chance.
"We tweaked things a little bit at half-time and we got a little better at controlling the game and were higher up the pitch and were a little more aggressive.
"We had to stay in the game, make sure Chelsea, who played well today, didn't get ahead. I don't think we played that well but because of our character and resilience that gave us a chance of being in the game."
As for whether Masuaku's winner was intended, Moyes added: "It was a great cross! We have to praise him for that, you have to get the ball in the box.
"It was a bit of a lucky goal, but you have to take what you get in football, and today we got that goal which made it 3-2."