Manchester United's positive week took a turn for the worse on Friday as striker Robin van Persie was ruled out for "around four to six weeks" with a sprained knee.
The Premier League champions announced the bad news on their official Twitter page, with the Holland international cut down just two days after firing the club into the quarter-finals of the Champions League.
Van Persie's hat-trick against Olympiacos had been viewed as a possible turning point in United's poor season, but he was later carried off on a stretcher with Friday's news meaning he may not play again until May.
He is almost certain to miss the Champions League last-eight tie with Bayern Munich, which he single-handedly set up.
A tweet from United read: "Following further investigation, Robin van Persie has a sprained knee, which will keep him out for around four to six weeks."
Manager David Moyes had earlier suggested that there could be bad news regarding the former Arsenal man.
"We're getting Robin re-scanned again today," the Scot had said.
"We're just not quite conclusive on everything we've seen yet. We want to just check that out and be sure.
"He won't be available for (Saturday's) game (against West Ham). But we'll try to give you some update when we can on the outcome of it.
"We've got a lot of games coming up and we need all our players fit and available."
As well as the West Ham and Munich games, Van Persie, who has scored 17 goals this season, will sit out the derby against Manchester City on Tuesday.
BREAKING: Following further investigation, Robin van Persie has a sprained knee, which will keep him out for around four to six weeks. #mufc— Manchester United (@ManUtd) March 21, 2014