Match Summary
The London club's hopes of qualifying automatically for the Champions League knockout stages were dealt a blow as Atalanta overturned a deficit to triumph in their home stadium.
The two teams entered this game with the same points tallies in the league phase of this year's competition, having secured three victories, drawn, and lost to sit on 10 points.
For large parts there was not much to choose between them in this encounter too, even though Chelsea got their noses in front in the first half when the Brazilian converted the captain's delivery.
But The Serie A side remained a threat going forwards, with former Charlton and Everton attacker creating problems for the defence.
Reece James shot wide in the after the break but Atalanta increased the pressure and justly levelled when former West Ham United striker Gianluca Scamacca a cross.
Alejandro Garnacho drove a strong effort straight at the goalkeeper but they scored the winner in the closing stages when the Blues defence backed off the midfielder, who made no mistake with a powerful shot.
Chelsea tried to salvage a draw in stoppage time but a excellent stop by the Atalanta goalkeeper kept out Pedro.
The result means Chelsea drop out of the top eight—the spots that progress automatically for the next round—to 11th with two league phase games remaining.
Atalanta, meanwhile, climb up to third, three points ahead of the Londoners.
This was a hugely disappointing outcome for Chelsea as even a draw was leaving them well placed to secure the top-eight finish they require to bypass the lottery of the play-offs.
But they let their concentration slip in their defensive duties with seven minutes left on the clock and Atalanta took full advantage.
The Italian side are in good form at the moment so this was always going to be a tough examination for the visitors, but they will be frustrated to be departing with nothing after securing the early advantage.
But the bigger picture is that this continues Chelsea's concerning streak of form in all competitions.
Since defeating the Spanish giants 3-0 at the close of November they are now winless in their last four matches.
Chelsea boss Maresca enacted five changes for this game as he persisted with his strategy of rotating players in and out of the starting 11.
The changes here came after he implemented the same number for the loss to Leeds and six for the draw with Bournemouth prior to the trip to Bergamo.
Maresca will see rotation as essential as he tries to manage players' minutes and compete both at domestically and in Europe, but he stated the central contingent of players stays the same for crucial games.
"I believe tonight [in] the starting lineup we had eight or nine players who featured against Tottenham, FC Barcelona, and Arsenal," he stated.
"If you see the five changes that we did compared to Bournemouth it is a change but it is the same eight or nine players playing in that match.
"The ones who played this evening are the individuals who feature in almost all the games."
The Blues' next European fixture sees them host Pafos on the coming Wednesday (20:00 GMT).
The Italian team, meanwhile, host Athletic Club at the same time.
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