Thomas Tuchel was left cursing the "big mistakes" made by Chelsea as they slipped to a 3-2 defeat at West Ham.
The Blues boss counted the cost of two blunders by goalkeeper Edouard Mendy as they were beaten for only the second time in the Premier League this season.
Chelsea were leading through Thiago Silva's header when Mendy made an almighty mess of a weak Jorginho backpass and ended up bringing down Jarrod Bowen, allowing Manuel Lanzini to equalise from the spot.
Mason Mount's sublime volley put Chelsea back in front, but Bowen pegged them back again before Mendy pushed a speculative Arthur Masuaku effort inside his own near post to gift West Ham a notable victory and cast further doubt on the Blues' title credentials.
Tuchel said: "I don't see that our performance dipped. I think the performance was enough to win the game but we gave easy goals away.
"We made big mistakes against Manchester United and again today, and if you do that at this level you cannot expect to get results.
"We are not good enough in the details at the moment. We need to care about the details, not start asking big questions or doubting the big picture. Without the big mistakes there was a big chance we win this game.
"Edouard is going through a rough period. OK his decision-making was not on the highest level with the first goal but we brought him to an awkward situation. He could have saved it but he did not.
"We are far, far away from blaming anybody but we need to admit that we are making too many crucial mistakes at the moment."
Victory over Chelsea is always welcome for West Ham, but this was all the sweeter after three matches without a win and cements their place in the top four in the 'best of the rest' position outside the title race.
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 today, I think we can play much better than we did," said Moyes. " I thought Chelsea played well but our character gave us a chance."
Masuaku's goal was his first in the Premier League, but did the Congolese wing-back mean it?
"It was a great cross!" smiled Moyes. "But you have to get the ball in the box, you have to try it.
"It was a bit of a lucky goal but in midweek against Brighton a VAR decision went against us, so you have to take what you get in football and we got that to win 3-2."