Real Sociedad winger Januzaj spent four years with the Red Devils from 2013-17, after initially joining the club's academy.
"For me, the biggest disappointment at this club was Adnan Januzaj," Butt told ESPN. "I don't think I've seen a player, probably since Ryan [Giggs], who was as good as that. He was unbelievable. In my eyes he should have gone on to be a world superstar.
"He's still playing professional football [at Real Sociedad in Spain] and he will have a good career. I'm sure he's a millionaire and he won't be going to bed worrying about what bills he's got to pay, but he should have been a superstar."