Match Summary
The London club's chances of securing a place automatically for the European Cup knockout stages were dealt a blow as their Italian opponents overturned a deficit to triumph in Bergamo.
Each club went into this fixture with identical points tallies in the league phase of this season's tournament, having secured three victories, drawn, and lost to be on ten points.
For large parts there was not much to separate them in this encounter too, even though Chelsea got their noses in front in the opening 45 minutes when the Brazilian finished Reece James' cross.
But Atalanta remained a threat going forward, with former Premier League attacker causing issues for the defence.
Reece James fired wide in the second half but the home team upped the intensity and deservedly levelled when former West Ham United striker Gianluca Scamacca a centering pass.
Alejandro Garnacho hammered a powerful shot straight at the goalkeeper but they got the winner late on when the Chelsea defenders backed off Charles de Ketelaere, who seized the opportunity with a low shot.
The visitors tried to rescue a draw late on but a brilliant save by Marco Carnesecchi kept out Pedro.
The outcome means Chelsea slip out of the top eight—the positions that progress automatically for the knockouts—to 11th with two league phase games left to play.
Atalanta, meanwhile, jump up to third place, three points ahead of the Londoners.
This was a extremely disappointing outcome for Chelsea as even a draw was leaving them well placed to achieve the top-eight finish they require to bypass the uncertainty of the play-offs.
But they lost focus in their defensive duties with seven minutes left on the clock and Atalanta capitalised completely.
The Italian side are in rich vein of form at the moment so this was always going to be a tough test for the Blues, but they will be annoyed to be leaving with nothing after securing the lead.
But the broader context is that this perpetuates Chelsea's worrying run of form in all competitions.
Since beating Barcelona 3-0 at the close of November they are now winless in their last four matches.
Manager Enzo Enzo Maresca made five rotations for this match as he continued his policy of rotating players in and out of the first team.
The rotations here came after he implemented the identical amount for the defeat to Leeds United and six for the tie with AFC Bournemouth prior to the journey to Atalanta.
The coach will see squad rotation as essential as he tries to manage players' fitness and compete both at domestically and in Europe, but he maintained the core group of players remains the unchanged for crucial fixtures.
"I think in this match [in] the first XI we had eight or nine players who featured against Tottenham, Barcelona, and Arsenal," he commented.
"If you see the five alterations that we did relative to the previous game it is different but it involves the same eight or nine players playing Barcelona.
"The players who played this evening are the individuals playing almost all the games."
The Blues' next European fixture sees them host Pafos on the coming Wednesday (8pm GMT).
The Italian team, on the other hand, are at home to Athletic Club at the same time.
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