The 1999 Champions League winner has previously spoken of his disappointment at Januzaj but elaborated on his unfulfilled potential.
"Not many come back and bite you on the backside," he explained. "They don't all go on to be superstars and make great careers, but at 17, 18 or 19 you tend to get the right idea.
"I felt Adnan should have been a world superstar. I remember speaking to Steve Bruce in the players' lounge before a game when Ryan [Giggs] took over [against Hull] and I was his assistant. I said, 'Adnan is as good as him,' and pointed at Ryan. He said 'no chance'. After the game Brucey said he was top, top drawer.
"That was a disappointment. He's gone on to have a career, so it's hard to say. But I think he could have been a superstar if he'd had the mentality of Ryan."