Almost 12 months since his last appearance, Fletcher returned to action as a second-half substitute in Sunday's 3-0 win at Aston Villa.
The Scotland midfielder showed no ill-effects from the outing afterwards, with Moyes clearly delighted to have the 29-year-old back.
"If anybody returns from an illness to play professional football on this kind of stage for a club like Manchester United, it shows a lot about the boy himself," said Moyes.
"He has had to be on his own, he has been in and out of hospital, he has had operations.
"Today was a big message to a lot of people - keep believing.
"If you are not well then keep believing that you can get yourself right."
Fletcher received huge applause from the visiting fans, both before and after his entrance.
"I always believed I would come back," he told MUTV.
"Other people around me were trying to make me think otherwise, but I stayed strong and believed I would get back.
"Playing a game like that, and getting an ovation like that, makes it all worthwhile."
But Moyes has refused to say whether Fletcher will be in contention for Wednesday's Capital One Cup quarter-final at Stoke.
"We got him three or four reserve games and he seemed to cruise through those so we had to get him back in the first team to see how he did," said Moyes.
"It is great for him to put the strip on and play because his career was in jeopardy."