James completed his move from Swansea City this week.
The Welshman had a season on-loan with the Shrews and Rodman recalled to Sky Sports: "I had never really seen pace like that before.
"Off the mark, wow. I remember playing against Antonio Valencia for Aldershot in the cup back when he was a young winger and his standing start was unbelievable.
"You knew he was going down the wing so you would stand a yard off him because of that but he would still drop the shoulder and beat you. You cannot stop it and Dan reminds me a bit of that.
"It was frightening. Scary. I saw it in a reserve game against Walsall. He was outstanding. Then there were the training sessions when he would just pick up the ball and do things that other people can't do.
"That's why clubs like United sign people, because they do things that others can't. If he gets one-on-one with someone in a foot race there is only one winner.
"If you have a footballing brain on top of that, and he obviously does given what he has shown this year, then it is no wonder that Manchester United have made a move for him.
"I guess I have to be grateful that he went back because if he had started showing some of the form that he has got now I would have been straight out of the team."