The midfielder's last competitive appearance came against Newcastle United on Boxing Day 2012.
An inflammatory bowel disease has kept him on the sidelines for the best part of a year, with the condition requiring another lengthy period of treatment.
Fletcher has, however, come through a 90-minute run-out for United's U21 side in recent weeks as he closes on full fitness.
The Scotland international is hoping to soon be back in contention for a place in Moyes' first team plans, but the United boss will not be rushing the 29-year-old back into the fold.
He told the club's official website: "I don't think he's quite ready yet.
"He has a bit to do in terms of more reserve games.
"It's great to see him training regularly, that's a big thing. He's beginning to look like a footballer again, and that in itself is a real plus for him.
"I wouldn't want to put a date on (a return) because of the condition. It's something we just have to take as we go along.
"He's training well and looking good, so we'll try to ensure that continues. I would hate to put a date on things and I'd need to check in with the doctors as well that everything is going to plan."