United had attempted to manage the condition that saw him miss the majority of last season, but failed and now Fletcher will undergo surgery.
Speaking of the player, Sir Alex said that it may take a change in his role to prolong his spell at Old Trafford when he returns from this latest setback:
“We tried to manage the condition with the treatments he was using.
“This is a blow for the boy but given he’s been dealing with the condition for a couple of years now. This is another step in terms of his health and his career. This will improve his prospects.
“He has had to change his game. When he played this season he wasn’t the Darren Fletcher we knew. He was sitting in front of the back four and he did that very well. That could be his role in the future. Some of his performances were very good. When he comes back – and I’m sure he will – it will be in a different role.”