Powell has impressed the Scot in his first season at Old Trafford.
Fletcher told the Manchester Evening News: "They are tough leagues and he has had experience of that. He is not afraid to put a tackle in and he uses his body well. He is a very good athlete.
"In some ways, it is a better grounding at the lower level because you get toughened up and hardened up as a youngster. You play with and against good experienced pros in the lower leagues, players who are going to kick you and you have to learn to survive.
"It may not be the grounding in terms of what you expect of a Manchester United player but it hardens you up. You are playing in games that matter so, mentally and physically, it is tough.
"We have got him at a great age where he is going to become a great Manchester United player.
"He is going to be a goalscoring midfielder for us and he will be a creative midfielder - there is no doubt about that. That comes naturally to him but the other off-the-ball stuff he is learning.
"He has a great eye for a goal and strikes the ball very well. I am not saying he is another Scholesy, because he is the best player I have ever played with, but let's hope he is the new Nick Powell and is himself and he will be a great player for Manchester United. I am sure he will be."