Match Summary
The London club's chances of securing a place automatically for the European Cup knockouts were dealt a blow as Atalanta came from behind to claim victory in Bergamo.
Each club went into this fixture with identical records in the league phase of this season's competition, having won, drawn, and suffered one defeat to be on 10 points.
For large parts there was very little to distinguish them in this encounter too, even though Chelsea went ahead in the opening 45 minutes when Joao Pedro converted Reece James' delivery.
But Atalanta consistently appeared a threat going forward, with former Charlton and Everton attacker creating issues for the defence.
James shot wide in the second half but the home team increased the intensity and rightfully drew level when former West Ham United striker Gianluca Scamacca a centering pass.
The winger drove a strong effort straight at the goalkeeper but they scored the decisive strike in the closing stages when the Chelsea defenders backed off Charles de Ketelaere, who seized the opportunity with a low finish.
The visitors tried to rescue a draw in stoppage time but a excellent save by Marco Carnesecchi denied Pedro.
The final score means Chelsea slip out of the top eight—the positions that qualify automatically for the next round—to eleventh with two league phase games remaining.
The Bergamo club, meanwhile, jump up to third place, three points ahead of the Blues.
This was a deeply disappointing outcome for Chelsea as even a point was leaving them well placed to secure the top-eight finish they need to bypass the uncertainty 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 fine fettle at the moment so this was always going to be a tough test for the visitors, but they will be disappointed to be departing with nothing after taking the lead.
But the broader context is that this perpetuates Chelsea's troubling run of results in all competitions.
Since defeating the Spanish giants 3-0 at the end of November they are now without a win in their last four games.
Manager Enzo Maresca made five alterations for this game as he continued his run of moving players in and out of the starting lineup.
The changes here followed after he implemented the same number for the defeat to Leeds and six for the tie with Bournemouth prior to the journey to Atalanta.
Maresca will see squad rotation as essential as he tries to manage players' workloads and challenge both at domestically and in Europe, but he maintained the central contingent of players remains the unchanged for crucial games.
"I believe in this match [in] the starting lineup we had eight or nine players who featured against Tottenham Hotspur, Barcelona, and the Gunners," he stated.
"If you see the five changes that we made relative to Bournemouth it may be a change but it is the eight or nine players playing in that match.
"The ones who took to the field this evening are the ones playing almost all the matches."
Chelsea's next European fixture sees them welcome Pafos on Wednesday, 21 January (8pm GMT).
The Italian team, meanwhile, host Athletic Bilbao at the identical kick-off time.
