The 29-year-old has not played for United since December 2012 due to a bowel problem and has been limited to appearances for the Under-21 side, completing a fixture earlier in November.
Moyes feels Fletcher needs to play more with the youth team before he can be considered for a return to the top squad.
"I don't think he's quite ready yet," Moyes told the club's official website. "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."
Moyes is also currently without midfielder Michael Carrick because of an Achilles injury but is eager to take his time with Fletcher's return because of the severity of ulcerative colitis - a form of inflammatory bowel disease.
"I wouldn't want to put a date on (a return) because of the condition," he added. "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."