Robin van Persie has been ruled out for "four to six weeks" with a sprained knee, Manchester United have confirmed.
Van Persie was carried off on a stretcher in the dying minutes of United's 3-0 win over Olympiakos, in which scored a hat-trick.
The Netherlands striker will miss the Champions League quarter-finals against Bayern Munich and could be sidelined for as many as six Premier League matches.
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 forward.
"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."