The teenager put in a fine performance in a 3-1 win for the Red Devils, with 18-year-old James Wilson scoring the first two goals on his United first-team debut.
"We see the young players they've got, Januzaj in particular. That kid's going to be a superstar, isn't he?" Bruce told the press after the game.
"If he keeps his feet on the ground, which I'm sure they will do here, then he's got everything, the boy."
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"In the second half we decided to have a go at it and in the first half we were very comfortable," the Hull boss told the press. "A lot of players have staked a claim to play in the FA Cup final.
"Matty Fryatt has been terrific. He changed the semi-final and every time I have asked him to come on he has done well. He looks favourite to take that position.
“Since we've got through [to the final], the biggest problem I've had is keeping everybody focused on the job," Bruce added.
"After Swansea [in the league, survival] was over and done and my hardest problem has been to keep everyone focused on what lies ahead and that's been very, very difficult.
"We need the final to come along quickly but, make no mistake, we'll be ready for it."
Bruce also paid tribute to his opposite number as Ryan Giggs came off the bench to inspire United in the closing 20 minutes on what could be his final Old Trafford appearance as a player.
"If it has ended for Ryan Giggs it has been some career," the Welshman's former United team-mate remarked. "There will never be anyone as decorated.
"Sometimes the word great is used too often but he has been one of the greats."