Jose Mourinho fears Manchester United defender Eric Bailly has suffered serious knee ligament damage.
The Portuguese manager endured a nightmare return to Stamford Bridge, suffering a humiliating 4-0 defeat and losing a key player in the process.
Bailly, who has impressed since his summer move from Villarreal, limped off after 52 minutes with what Mourinho believes could be a long-term knee injury.
"He is injured and I am afraid he is badly injured," the United manager said. "In his knee, in the ligament area. He feels that it is really bad."
Bailly has played in all but one of United's matches this season, winning the club's Player of the Month award for August.
The centre-back was Man of the Match in three of their four games that month but could now face a substantial period out injured.
There are also concerns over Marouane Fellaini, caught by a nasty-looking David Luiz challenge.
The Chelsea defender was booked and the Belgium midfielder was replaced at half-time.
Mourinho would not be drawn on whether he felt the referee's decision was correct and said little when asked if Fellaini was injured by the challenge.
"If I'm going to say that he was injured, you are going to say again..." he said. "It [the substitution] was a bit of both [tactical and injury]."
United are back in action on Wednesday when Manchester City arrive at Old Trafford in the EFL Cup, with a home match against Burnley following on Saturday.
Wayne Rooney looks a doubt for those games after missing the Chelsea match with an unspecified injury.
"Injured in training in the last session," Mourinho told Sky Sports when asked about his captain's absence at Stamford Bridge.
"He was so keen to play against Fenerbahce that he hid it and then he felt it in the training session and he couldn't come."