Match Summary
The London club's chances of securing a place automatically for the European Cup knockout stages were dealt a blow as Atalanta fought back to claim victory in their home stadium.
Each club went into this match with identical points tallies in the league phase of this year's competition, having secured three victories, drawn, and lost to have accrued ten points.
For much of the game there was very little to separate them in this encounter too, although Chelsea got their noses in front in the first half when the Brazilian finished the captain's delivery.
But Atalanta remained a threat going forwards, with ex- Premier League attacker causing problems for the defence.
Reece James fired wide in the second half but the home team upped the intensity and justly drew level when former West Ham United striker Gianluca Scamacca a cross.
Alejandro Garnacho hammered a strong effort straight at the goalkeeper but they secured the winning goal in the closing stages when the Blues defence retreated from the midfielder, who made no mistake with a powerful shot.
Chelsea attempted to rescue a point late on but a brilliant stop by the Atalanta goalkeeper denied Pedro.
The final score means Chelsea drop out of the top eight—the positions that progress automatically for the knockouts—to 11th with two group stage matches left to play.
The Bergamo club, meanwhile, climb up to third, three points ahead of the Blues.
This was a hugely disappointing outcome for Chelsea as even a point was leaving them in a strong position to achieve the top eight finish they need to avoid the lottery of the play-offs.
But they lacked concentration in their defensive duties with seven minutes left on the clock and their opponents took full advantage.
The Italian side are in fine fettle at the moment so this was always going to be a difficult test for the Blues, but they will be disappointed to be departing with no points after taking the lead.
But the broader context is that this perpetuates Chelsea's concerning streak of form in all tournaments.
Since defeating Barcelona 3-0 at the end of November they are now without a win in their last four matches.
Chelsea boss Enzo Maresca enacted five rotations for this match as he persisted with his policy of moving players in and out of the starting 11.
The rotations here came after he implemented the same number for the loss to Leeds United and six for the draw with AFC Bournemouth prior to the trip to Bergamo.
The coach will view rotation as vital as he attempts to handle players' fitness and challenge both at home and abroad, but he maintained the key nucleus of players remains the same for important fixtures.
"I think in this match [in] the starting lineup we had eight or nine players who played against Tottenham Hotspur, Barcelona, and the Gunners," he stated.
"If you look at the five alterations that we did compared to the previous game it is different but it is the eight or nine players who played Barcelona.
"The players who played this evening are the individuals playing almost all the games."
The Blues' next Champions League fixture sees them host Pafos on the coming Wednesday (20:00 GMT).
Atalanta, meanwhile, are at home to Athletic Bilbao at the same time.
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