The Scottish midfielder last appeared against Newcastle on Boxing Day as United beat Alan Pardew’s side at Old Trafford, but that would be his 10th and final outing for the club this season.
United had attempted to manage the condition that saw him miss the majority of last season, but failed and now Fletcher will undergo surgery.
“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.”