He visited Dr Lars Petersson, in Gothenburg on Sunday, who confirmed the pain Ole experienced in pre-season training was due to damage in his knee cartilage.
"It will take at least one year before Ole can play football again," said Petersson.
"There is a good chance of him playing football again, but I cannot give any guarantees."
A United spokesman said:
"Ole underwent an arthroscopy and this unfortunately confirmed persistent damage to the articular cartilage. He will undergo further treatment but it is likely he will miss the whole of this season."
"It's just unfortunate that a player of that calibre and such a great professional is missing out the best years of his life as a player. It's only in the last two or three years he became a consistent first-team performer. He was always the player we brought off the bench because he was so good at it. He was probably the best sub of all time, a player who could analyse football just by watching it and understood his role."
Most of the press speculate that we will now try and speed up any bid for Rooney, and not wait until January.