Fletcher was restricted to 13 appearances last season as he continues to battle ulcerative colitis - a debilitating bowel condition.
The 29-year-old had a third surgery earlier this week and will now begin rehabilitation in a bid to return next season, according to The Mirror.
He told The Mirror: "It's been a hard time the last couple of years dealing with something like this. As a professional footballer all you want to do is play, but that's not been possible for me.
"But if the operation is a success I've been told there should be nothing to stop me being cleared to play again."
United boss David Moyes will monitor the situation closely and reportedly hopes to reintroduce Fletcher in the next few months.