Jose Mourinho confirmed Eric Bailly sustained a knee ligament injury during Manchester United's 4-0 defeat to Chelsea on Sunday.
Bailly collided with Eden Hazard early in the second half and hobbled off to be replaced by Marcos Rojo in the 52nd minute.
Mourinho fears it could be a "bad" injury, with the Ivorian centre-back now facing a lengthy spell on the sidelines.