Chelsea fear Maxence Lacroix could be sidelined until January after the defender suffered a serious hamstring injury during Wednesday’s Carabao Cup defeat by Liverpool.
Multiple sources have told BBC Sport that the France international, 26, faces a potentially lengthy recovery and could miss up to 10 matches.
Lacroix was injured towards the end of the first half while sprinting to block a shot in a match Chelsea eventually lost on penalties.
Manager Xabi Alonso has not confirmed the full extent of the problem but ruled the centre-back out until after the international break.
“Max, he picked up an injury, and we expect that he’s not going to be ready before the international break,” Alonso said before Saturday’s Premier League meeting with Manchester United.
“So he will be a few weeks out.”
The setback deprives Alonso of a player who has quickly become central to his plans following a £52m summer move from Crystal Palace.
Lacroix has started all but one of Chelsea’s 11 matches across all competitions this season, with only attacker Morgan Rogers playing more minutes.
Wesley Fofana, Levi Colwill and Josh Acheampong offer alternatives, although fellow centre-backs Jorrel Hato and Aaron Anselmino are currently injured.
Alonso backed Wes Fofana to make an important contribution despite his penalty miss against Liverpool.
“He (Fofana) was consistent, he was strong and I saw many positives from his performance,” Alonso said.
“We know the level that he (Fofana) has had and he’s going to be important.”
Meanwhile, goalkeeper Emiliano Martinez is having a protective mask fitted as he recovers from a broken jaw.
Midfielder Moises Caicedo avoided injury despite concerns following the Liverpool match.
Chelsea enter the Manchester United fixture fifth in the Premier League, having collected 13 points from eight games.
They have kept just one clean sheet during that run, underlining the defensive challenge ahead.
