Powell, who was handed his pro debut as a 16-year-old by Gradi in 2010, was one of the country's hottest prospects when he joined United from the Alex in 2012.
"He still has time to have a very good career," Crewe director of football Gradi told BBC Stoke.
"It's a shame he has been so unlucky with injuries but he is still young.
"He may have to adjust and adapt his game but he is a match-winner.
"I know he's struggled physically over the last few seasons but if a club can get him fit and running around again, then he has incredible ability.
"If he'd remained injury free and [Sir] Alex [Ferguson] had stayed at Manchester United then I'm sure he would have been challenging for an England place.
"He is certainly good enough."