Harrison scouted Powell for Blackburn Rovers before United signed him earlier this summer.
He told the Manchester Evening News: "It makes no difference at all that he's come from League Two.
"If you look back over the years, players like Kevin Keegan went from Scunthorpe to Liverpool and you can go through a host of players who have done the same.
"It doesn't matter who you have played for if you are good enough.
"I only had to watch him twice before I told Blackburn to sign him.
"When I read that United had signed him I thought 'He's going to be a terrific player'. I'm certain about that.
"He looks confident and I don't think a move to United will phase him at all.
"It's nice to see them signing young players.
"He's in a similar bracket to those guys."