At a Glance
Chelsea's chances of securing a place automatically for the European Cup knockouts were dealt a blow as their Italian opponents overturned a deficit to triumph in their home stadium.
The two teams went into this match with identical records in the initial stage of this year's tournament, having secured three victories, drawn, and suffered one defeat to have accrued 10 points.
For much of the game there was very little to separate them in this contest too, even though Chelsea went ahead in the opening 45 minutes when Joao Pedro turned in the captain's delivery.
But The Serie A side remained a threat going forwards, with former Premier League attacker Ademola Lookman problems for the backline.
Reece James fired wide in the after the break but the home team upped the pressure and rightfully equalised when ex-West Ham striker Gianluca Scamacca a centering pass.
The winger drove a strong effort straight at the goalkeeper but they scored the winner late on when the Blues defence backed off Charles de Ketelaere, who took full advantage with a driven finish.
The visitors attempted to salvage a point late on but a excellent save by the Atalanta goalkeeper kept out Joao Pedro.
The outcome means Chelsea slip out of the top eight—the positions that progress automatically for the knockouts—to 11th with two group stage games remaining.
The Bergamo club, meanwhile, jump up to third place, three points ahead of the Blues.
This was a hugely frustrating result for Chelsea as even a point was leaving them well placed to secure the top eight position 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 seized the moment.
The Italians are in rich vein of form at the moment so this was always going to be a tough test for the Blues, but they will be frustrated to be departing with nothing after securing the early advantage.
But the bigger picture is that this perpetuates Chelsea's concerning sequence of form in all tournaments.
Since defeating the Spanish giants 3-0 at the close of November they are now without a win in their last four games.
Chelsea boss Maresca made five changes for this match as he continued his policy of moving players in and out of the first team.
The changes here followed 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 Atalanta.
The coach will view rotation as vital as he attempts to handle players' workloads and challenge both at home and in Europe, but he maintained the central contingent of players stays the unchanged for important fixtures.
"I think tonight [in] the first XI we had eight or nine players who played against Tottenham, Barcelona, and Arsenal," he stated.
"If you see the five alterations that we made relative to Bournemouth it is different but it involves the eight or nine players who played in that match.
"The players who took to the field tonight are the individuals playing almost all the matches."
The Blues' next European fixture sees them host the Cypriot side on Wednesday, 21 January (8pm GMT).
The Italian team, meanwhile, host Athletic Club at the same time.
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