At a Glance
The London club's hopes of securing a place automatically for the Champions League knockout stages were suffered a setback as their Italian opponents came from behind to triumph in Bergamo.
Both sides went into this match with identical records in the league phase of this season's tournament, having won, tied one, and suffered one defeat to be on 10 points.
For extended periods there was very little to distinguish them in this contest too, even though Chelsea took the lead in the first half when the Brazilian turned in Reece James' delivery.
But Atalanta consistently appeared a danger going on the attack, with ex- Charlton and Everton attacker Ademola Lookman issues for the defence.
James fired wide in the second half but the home team upped the intensity and justly equalised when ex-West Ham striker headed in a centering pass.
The winger drove a powerful shot straight at the goalkeeper but Atalanta got the winning goal in the closing stages when the Blues defence retreated from Charles de Ketelaere, who made no mistake with a powerful shot.
Chelsea tried to rescue a point late on but a brilliant stop by Marco Carnesecchi denied Joao Pedro.
The outcome means Chelsea drop out of the automatic qualification places—the spots that qualify automatically for the knockouts—to eleventh with two group stage matches remaining.
The Bergamo club, meanwhile, jump up to third place, three points ahead of the Londoners.
This was a deeply frustrating result for Chelsea as even a point was leaving them in a strong position to achieve the top-eight position they need to bypass the uncertainty of the qualifying play-off round.
But they lost focus in defence with seven minutes remaining and Atalanta seized the moment.
The Italians are in rich vein of form at the moment so this was always going to be a difficult examination for the Blues, but they will be annoyed to be departing with nothing after taking the lead.
But the bigger picture is that this perpetuates Chelsea's concerning run of results in all competitions.
Since beating Barcelona 3-0 at the end of November they are now winless in their last four matches.
Manager Enzo Enzo Maresca made five changes for this match as he persisted with his run of rotating players in and out of the first team.
The rotations here followed after he implemented the same number for the loss to Leeds and six for the draw with AFC Bournemouth prior to the journey to Bergamo.
The coach will see rotation as vital as he tries to manage players' fitness and challenge both at home and in Europe, but he maintained the central contingent of players stays the unchanged for important fixtures.
"I think in this match [in] the starting lineup we had eight or nine players who played against Tottenham, Barcelona, and Arsenal," he commented.
"If you look at the five changes that we did relative to the previous game it may be different but it involves the eight or nine players who played Barcelona.
"The ones who played tonight 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, on the other hand, host Athletic Club at the same time.
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