Chelsea's hopes of a top-eight finish in the Champions League took a blow as they were beaten 2-1 by Atalanta thanks to a late goal from Charles De Ketelaere.
The Serie A side have lost only once at home this season, while Enzo Maresca's side are yet to win away in Europe, having now lost twice and drawn once. They are now winless in their last four games in all competitions.
Atalanta began brightly and would have gone ahead were it not for a sensational block from Josh Acheampong to deny Ademola Lookman on his first Champions League start.
Chelsea made the initial breakthrough in the 25th minute. Reece James' fizzing cross was finished by Joao Pedro, but the offside flag immediately went up.
The forward wagged his finger in disagreement - and he was right. VAR stepped in and found both James and Joao Pedro were onside, and the goal was given.
Atalanta had not scored in the first half of a European game this season, and while they couldn't end that particular run they did find the net shortly after half-time - but not before James missed a huge chance to double Chelsea's lead.
The hosts equalised when De Ketelaere sent in a superb cross for former West Ham striker Gianluca Scamacca to nod home.
Atalanta maintained their pressure, and always looked the most likely to score another. The in-form De Ketelaere had a slice of luck with his winner too as his shot deflected off Marc Cucurella - who was backing off the forward - and fizzed through the outstretched hand of Robert Sanchez.
It is the second game in a row the Belgian has registered two goal involvements, with Atalanta now up to third in the Champions League table.
Chelsea saw late chances from Alejandro Garnacho, James and Joao Pedro go begging, and as a result, they drop out of the top eight. They are now 11th, two points from the automatic round-of-16 spots.
Chelsea head coach Enzo Maresca:
"First half we were winning, we were good controlling the game. Probably the two goals we conceded in a short time, two easy goals that we can both avoid.
"After the first goal, we lost a little bit of control of the game. Also, we are playing every two days. We look like we struggle a little bit. We also tried to be aggressive.
"We had different chances also to score for 2-0, especially with Reece [James]. And when we conceded the 1-1, the dynamic of the game changed a little bit."
On their chances of making the top eight: "Probably with two wins, probably with 16 points, you can be in the top eight.
"I'm not sure about that, but... if we want to try to finish top eight, we need to win both. Otherwise, we try to play the play-off and then go to the next round.
"But now that the game is finished, the focus has to be on Saturday's game, because now we need as soon as possible to win games."
On making five changes from the draw at Bournemouth: "Tonight, the XI we had inside the pitch, eight or nine players played Tottenham, against Barcelona, against Wolves, Arsenal.
"They are the ones that they are playing every time, these kind of games, so if you see the five changes that we did compared to Bournemouth, it's different."
