Robin van Persie is out for up to six weeks and will miss Manchester United's UEFA Champions League quarter-final with Bayern Munich.
Van Persie took a blow to the back of his left knee late in Wednesday's game against Olympiakos after scoring the hat-trick which secured United's place in the last eight.
And David Moyes must now go into that clash against the European champions without his top scorer after further examination revealed the extent of the injury.
Van Persie was initially ruled out of this weekend's Premier League trip to West Ham pending a second scan of the knee.
And a statement from United on Friday evening read: "Following further investigation, Robin van Persie has a sprained knee, which will keep him out for around four to six weeks."
According to that prognosis, the best-case scenario would see Van Persie miss both legs of the Bayern tie and Premier League games against West Ham, Manchester City, Aston Villa and Newcastle.
A six-week absence would rule the Dutchman out of a Champions League semi-final should United get that far without him.