Manchester United striker Ole Gunnar Solskjaer could be out for three months after suffering a recurrence of his long-standing knee problem.
Solskjaer missed five month of last season with knee trouble
The 30-year-old striker stayed home from United's pre-season tour of the United States for treatment.
An operation failed to solve the problem, and United boss Sir Alex Ferguson admitted his frustration.
"It's unfortunate a player of his calibre is missing out on the best years of his life," he said.
"It is only in the last two or three years he has become a consistent performer for us.
He was always the player we brought off the bench because he was so good at it.
"He was probably the best substitute of all time. He could analyse football by watching it, understood exactly his role when he came on, understood the tactics of the game.
"He is a rare bird."
Earlier, Solskjaer dismissed claims that injury could hasten the end of his Manchester United career.
"I don't feel any more threatened now than before. But I still have a lot to offer this club and I am determined to prove that," he told the Sun newspaper.
He added: "I am going to have to try harder than ever to get into the team,
"But that is how it is at United, you have to keep improving year on year.
"The standards keep being raised and you have to go with them."
United open their ChampionsWorld Series on Sunday when they face Bayern Munich