"Wilson is better than all of them," he told Tribalfootball. "To be fair he had a chance, he had his first game with Ryan and scored two goals.
"He's a finisher and he's got two feet. He's the best 15-year-old I've seen. He was great at scoring goals, he very rarely missed. He did it in the reserves and he can tear teams apart.
"I just hope he does well now. He's got to do something at Sheffield United now because he's a special player.
"I was with the academy for seven years. [James] Wilson was the best talent. I had an argument with [Man Utd youth coach] Colin Little, and he was saying Marcus [was better]. I said if you put them together now I know who will score more goals.
"But you've got to put him the right system to score goals, he needs to be running at people.
"He came in with Louis van Gaal and he was playing by himself and he is not really a target man. He's one of them you need to get on the ball and let him run with it.
"He played on his own in the reserves sometimes and I remember he played against [Manchester] City U23s in one game. In 30 mins, he hadn't touched the ball. When he finally dropped off the ball, he scored four goals."
Wilson scored twice on debut as an 18-year-old in 2014 while Ryan Giggs was caretaker manager. Louis van Gaal handed him 17 appearances the following season, before he tested by spending seven months on loan with Brighton.
The 22-year-old, who represented England at the U16, 19, 20 and 21 levels, held high hopes after he joined Derby on loan last season. However, a serious knee injury ruled him out for the season.
He has scored one goal in four Championship appearances for the Blades. He'll remain at Bramall Lane until the end of the season.