Ole Gunnar Solskjaer will be out for several weeks after hurting a hamstring in Manchester United's Champions League defeat by FC Copenhagen on Wednesday.
The striker had only just returned from a two-year lay-off with knee problems.
United manager Sir Alex Ferguson told the club website: "It's unfortunate for Ole but I am actually quite happy.
"Given his previous history of knee problems, I was a bit concerned but, fortunately, it is only a hamstring. He will be out for a few weeks."