LISANDRO MARTINEZ will be out for two months after suffering a ligament injury against West Ham.
The Argentine, 26, went down in a tangle with Vladimir Coufal on Sunday.
He tried to play on but collapsed soon after and was replaced by Raphael Varane.
It is NOT an ACL injury, but he has sprained his collateral ligament.
Man Utd hope to have Martinez back after the international break in March.
This means he is set to miss Premier League games against Aston Villa, Luton, Fulham, Manchester City, Everton and Sheffield United, as well as their FA Cup fifth round clash with Nottingham Forest or Bristol City.
The South American centre-back only returned from injury last month after a lengthy period out.
When asked about Martinez's condition after the game, Erik ten Hag was quoted as saying the defender "doesn't look well" and that it is "very bad for him, very bad for the team".
He went on to add: "It seems bad, we have to wait and make the right diagnosis.
"In this moment we are very sad, hopefully it is not too bad but we can only pray.
"It looks like he has to sit some games out injured again."
Martinez started the clash with West Ham alongside Harry Maguire as Raphael Varane was dropped to the bench.
He suffered a foot injury back in September that ruled him out until the new year.
He returned as a substitute in Man Utd's 2-2 draw with Tottenham in mid-January.
After playing in the win over Newport in the FA Cup, he started his first Premier League game in four and a half months against Wolves as his side sealed a 4-3 victory.
But there were concerns at Molineux when Martinez was spotted having his foot bandaged after coming off in the latter stages.
And now he faces yet more time on the sideline.