Robin van Persie faces at least two weeks on the sidelines with an ankle injury and will miss Manchester United's FA Cup quarterfinal against his former club Arsenal.
The Netherlands international suffered the injury during the 2-1 defeat at Swansea last weekend, leaving the Liberty Stadium on crutches.
United manager Louis van Gaal told a news conference: "He is injured. It is an ankle problem. I do not think it is very heavy, but ankle problems can take a long time -- it is not one or two weeks."
It means United are without Van Persie for Saturday's Premier League clash at home to Sunderland and Wednesday's trip to Newcastle.
He could also be out for March's pivotal Premier League clashes with Tottenham and Liverpool.
But vice-captain Michael Carrick is fully fit after a muscle problem while James Wilson, who suffered a dead leg playing for the under-21 team on Monday, is in contention to face the Black Cats.
Van Gaal added on MUTV: "Now Robin van Persie is the only injured player because [Michael] Carrick trained yesterday for the first time with the group.
"But he has an ankle injury now, so we will have to wait and see how long that shall take. James Wilson is available."
#INSERT type:image caption:Manager Louis van Gaal said ankle problems 'can take a long time -- it is not one or two weeks.' END#
Van Persie, who has scored 10 times for United this season, only missed only one of their last 25 league games.
His absence means Wayne Rooney, who has played in attack in United's last two games after a two-month spell in midfield, could be set for an extended spell as a forward.
Van Gaal's other striking options are Radamel Falcao, who has scored just four goals in 19 games for the club, and Wilson, who has only scored three league goals in his career.
The manager said: "The possibility [of bringing in Falcao] is higher of course, because when a striker is away another has to play. We will have to wait and see."
He could also opt to use midfielder Marouane Fellaini in a more advanced role.
Van Gaal said he was pleased to have Carrick, who has not played since January's 0-0 draw at Cambridge, available again.
"He is very experienced and he can play in a very important position, but he also needs match rhythm," he said. "He trained only one time with us, with the group, so he needs to also play in the second team."