Eric Bailly has suffered medial ligament damage - ruling him out for rest of the season and the Africa Cup of Nations.
The Manchester United defender underwent scans on Monday - which confirmed the extent of the damage - after suffering the injury in the 1-1 draw with Chelsea.
The Ivory Coast international left Old Trafford on crutches with his leg in a brace.
It is the latest blow for Bailly in a season decimated by injury.
It also raises further doubts about his United future at a time when Ole Gunnar Solskjaer plans to overhaul his squad.
A top class centre-back is among his priorities - but that would mean offloading at least one of his current roster of players in that position.
Chris Smalling, Phil Jones and Marcos Rojo have all recently signed long-term deals. Meanwhile, Victor Lindelof is seen as the long-term answer in that role.
Bailly would have been an obvious candidate to sell, particularly with his stock remaining high across Europe.
But it remains to be seen how long it will take him to recover and if the injury puts off potential suitors.
Kalidou Koulibaly is a long-term target for United, while they are monitoring Raphael Varane at Real Madrid.
Harry Maguire is another player highly thought of at Old Trafford.
