Head coach Graham Potter has acknowledged that he has concerns over the severity of the hamstring injury sustained by Ben Chilwell during Wednesday's 2-1 victory over Dinamo Zagreb.
Raheem Sterling and debutant Denis Zakaria got on the scoresheet as the Blues overturned an early deficit to record their 100th win in the Champions League.
However, a successful night ended on a low note as Chilwell pulled up sharply with a hamstring injury in the third minute of added-on time.
While the left-back will almost certainly miss the upcoming triple-header with Arsenal, Manchester City and Newcastle United, the England international is now a major doubt for the World Cup which begins later this month.
Although Potter wants to wait until Chilwell has undergone a scan, the Chelsea boss admitted that it was "not looking positive".
Speaking to reporters at a press conference, Potter said: "Not great. When he pulls up like that it's a concern. We need to scan but it's not positive at the moment."
Later in the media briefing, Potter added: "He didn't play at the weekend so there was a thought he could play tonight and have enough time to recover. We've tried to manage him. He's not played three games in a week and has had the build up that he needs. It's frustrating and it's a blow for him."
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Before that setback for club, player and the Three Lions, Zakaria enjoyed a strong 70 minutes on his first appearance since joining on loan from Juventus in September.
As well as a cool finish for his goal, the Switzerland international often combined well with Jorginho before being withdrawn with what Potter suggested was a minor "contact" injury.
Commenting on Zakaria, the Englishman said: "He's handled himself really well. Supported his team and teammates in a good way. Credit to him, everyone was so happy for him. It's a great story for a whole group I think."
While Chelsea had already secured top spot in Group E, their success in West London ensured that they finished three points clear of AC Milan.
Chelsea will now start to prepare for Sunday's London derby with Arsenal before difficult trips to play Manchester City in the EFL Cup and in-form Newcastle United in the top flight.
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