At a Glance
Chelsea's hopes of securing a place automatically for the Champions League knockouts were dealt a blow as their Italian opponents came from behind to claim victory in their home stadium.
Both sides went into this match with the same records in the league phase of this season's competition, having won, drawn, and lost to sit on ten points.
For much of the game there was very little to separate them in this encounter too, even though Chelsea got their noses in front in the first half when Joao Pedro turned in the captain's delivery.
But The Serie A side remained a threat going forward, with former Premier League attacker Ademola Lookman issues for the defence.
Reece James fired wide in the second half but the home team upped the intensity and justly levelled when ex-West Ham striker Gianluca Scamacca a cross.
Alejandro Garnacho hammered a strong effort straight at the goalkeeper but Atalanta secured the winning goal late on when the Blues defenders backed off the midfielder, who made no mistake with a driven finish.
Chelsea tried to salvage a point late on but a brilliant stop by Marco Carnesecchi kept out Pedro.
The outcome means Chelsea fall out of the top eight—the positions that progress automatically for the next round—to eleventh with two group stage matches left to play.
Atalanta, in contrast, leap up to third place, three points clear of the Blues.
This was a hugely disappointing outcome for Chelsea as even a point was leaving them in a strong position to secure the top eight finish they need to bypass the lottery of the play-offs.
But they lacked concentration in defence with seven minutes left on the clock and their opponents took full advantage.
The Italians are in good form at the moment so this was always going to be a difficult test for the visitors, but they will be disappointed to be departing with nothing after securing the lead.
But the wider issue is that this continues Chelsea's troubling run of results in all competitions.
Since defeating the Spanish giants 3-0 at the end of November they are now winless in their last four games.
Chelsea boss Enzo Maresca enacted five alterations for this game as he persisted with his strategy of rotating players in and out of the starting lineup.
The changes here came after he made the identical amount for the defeat to Leeds and six for the draw with Bournemouth prior to the journey to Bergamo.
Maresca will view squad rotation as essential as he tries to handle players' workloads and challenge both at home and in Europe, but he maintained the key nucleus of players remains the unchanged for key games.
"I believe in this match [in] the starting lineup we had eight or nine players who played against Tottenham, Barcelona, and Arsenal," he stated.
"If you see the five alterations that we did relative to Bournemouth it may be a change but it involves the same eight or nine players playing in that match.
"The players who played tonight are the ones who feature in almost all the matches."
The Blues' next Champions League fixture sees them host the Cypriot side on Wednesday, 21 January (20:00 GMT).
Atalanta, meanwhile, are at home to Athletic Club at the same time.
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Thomas Pineda
Thomas Pineda
Thomas Pineda
Thomas Pineda
Thomas Pineda
Thomas Pineda