The local lad was unlucky to be ruled out of the Under-21 league final against Chelsea at Old Trafford this week through injury, having formed a superb central-midfield partnership with pal Ben Pearson.
"It's unfortunate Joe missed out because he's got great energy and has been the one that's really surprised me in the Under-21s this season," explained Fletcher to MUTV.
"He's improved all year and, by coming up against him in training, I know how much quality he has got. Once he develops physically, I think we're going to have a good player on our hands."
United's Jesse Lingard recently revealed how Sir Alex Ferguson told him to be patient, with the young forward stating: "He sat down with me and my dad when he was manager and said that I'd probably be ready for the first team at 22." Fletcher feels a long-term policy will also be adopted with Rothwell.
"Joe's a little bit like Jesse Lingard," said Fletcher. "I know the [former] manager spoke about him and said we might not see the best of Jesse in the first team until maybe his early 20s. I think we've got the same situation with Joe."