The Scotland international has only featured on 13 occasions since he took an extended break from the game after he was diagnosed with ulcerative colitis in November 2011.
The 29-year-old recently underwent surgery in a bid to speed up his chances of turning out for the Red Devils again, but at the moment, there is no indication as to when that may happen.
"I'm still in the hands of the surgeons at the moment - I've not actually started training yet," PA quotes him as saying.
"I've been doing my own little bit in the gym, which is keeping me ticking along nicely and I'm waiting for the surgeon to give me the go-ahead to join in training. I don't have a timescale for when I will be back playing really.
"Hopefully it is not going to be too far away in the distant future. I just need to be patient. After all I've gone through I think it's important that I'm patient. If it means a few extra weeks to get myself right, then so be it."
Fletcher signed on at United as a trainee in 1995.