Chelsea are set for a nervous wait to establish the extent of an injury that Denis Zakaria picked up during Thursday's 2-1 defeat to Fulham.
Former Chelsea winger Willian gave Fulham a half-time lead, before Kalidou Koulibaly equalised in the opening stages of the second period.
However, the Blues then experienced a disastrous couple of minutes just before the hour mark when January signing Joao Felix was shown a straight red card on his debut moments after Zakaria had been forced off through injury.
With Chelsea down to 10 men, they were unable to prevent Fulham from clinching all three points courtesy of a Carlos Vinicius header 17 minutes from time.
The result represented a third consecutive defeat for Chelsea, while they have also won just two of their previous 10 competitive games.
As well as being disappointed with the loss to their West London rivals, Chelsea will be concerned about the extent of Zakaria's injury after he went down under no contact.
Under-pressure Chelsea boss Graham Potter indicated after the game that it was an issue with Zakaria's quadricep and that he would need to undergo further assessment.
Potter told reporters: "The only assessment is it's at the top of the quad. He felt it when he cleared the ball. In terms of the extent, it's too early to say."
Although Zakaria failed to feature in the top flight in the first half of the season after joining on loan from Juventus last summer, he had started to establish himself in the Chelsea lineup following the World Cup break.
The 26-year-old, who has a purchase option in his loan deal, has started Chelsea's last four Premier League games, but he could now be set for a spell on the sidelines.
Zakaria's injury will come as a blow to Potter, who already has a number of players on the sidelines, while Felix will serve a three-match suspension.
The Portugal international will miss Premier League games against Crystal Palace, Liverpool, and the return meeting with Fulham.
Since the start of January, Christian Pulisic, Raheem Sterling and Pierre-Emerick Aubameyang have all sustained injuries.
Chelsea have also had to make do without their first-choice full-backs, Reece James and Ben Chilwell, while Edouard Mendy, N'Golo Kante, Wesley Fofana, Ruben Loftus-Cheek and Armando Broja are all sidelined.
The Blues, who sit in 10th place in the Premier League table, will try to return to winning ways when they face Crystal Palace on Sunday.
