Jenas played with Bale at Spurs before Wales captain left for Real Madrid.
"He's come back in a different body by the looks of it," Jenas said. "I remember being at Tottenham with Gareth Bale and he did something similar one summer.
"He turned up one summer and you were like 'Who is this person?! he turned into a man all of a sudden.'
"I think the biggest thing with Mason Greenwood is his finishing is just lethal. They are few and far between, strikers like that in today's game, so Manchester United have stumbled across something special."