The Scotland skipper has played just 13 games since announcing he was taking an extended break from football in November 2011 when he was diagnosed with ulcerative colitis.
The 29-year-old has undergone numerous operations and been put on a strict dietary regime in a bid to correct his stomach condition and allow him to resume playing duties.
But a definitive date for his comeback still eludes him and his medical advisors.
Now Fletcher - who will miss the start of the Barclays Premier League campaign, including United's first match against away to Swansea on August 17 - has made the decision to stay away from David Moyes' squad until he is ready to rejoin them on a permanent basis.
He said: "I'm staying away from the training ground right now just because it is difficult being there when you are not actually training.
"I've spoken to the manager, he seems great and we have discussed the plan going forward. I just can't wait to go back when I'm ready to get back training, not just plodding around no use to anyone."
Fletcher last went under the surgeon's knife in January but while a timescale has not yet been placed on his return to action, he believes there is finally light at the end of the tunnel.
"It's been very frustrating," he said. "I've struggled for three years. But knowing that it is hopefully coming to an end and that I can get back playing football to the level I expect to be playing is exciting.
"I've got to believe that will be the case and I do. I've not been told there is a reason why that won't be the case, so positive thinking is the only way I can get through these situations.
"I'm still in the hands of the surgeons at the moment - I've not actually started training yet.
"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."
Moyes took over as United boss this summer following the retirement of Sir Alex Ferguson.
Fletcher has already had discussions with the former Everton manager about how best to manage his condition and has also held similar talks with new Scotland boss Gordon Strachan.
And the player stressed both men are giving him their full support.
He said: "I've had a chat with the new manager at United that will remain between the two of us. It was a positive conversation and he's been very much in favour of letting me get on with my recovery. When I'm ready to come back, that's when he will look at me in terms of the playing side.
"It's the same with Gordon Strachan. I've been in touch with him. He's been very positive and enthusiastic."
Fletcher's last appearance for Scotland was in the 2-0 World Cup qualifying defeat to Belgium last October.
Since then the two defeats in the three Group 1 fixtures that followed his last outing have counted the Dark Blues out of the race for next year's finals in Brazil.
The side travel south to face England at Wembley on August 14 and the national side's captain admits missing out on that match is just as painful as watching his team-mates suffer on the international stage.
He said: "It's always difficult watching the team lose - but it's difficult watching them win because you want to be part of it.
"I wish I was going to Wembley playing but in these circumstances I've just had to think about myself and try and get back to where I want to be."