The Belgian, who was handed his first Premier League start at the Stadium of Light, scored both of United's goals as the champions came from behind to record a 2-1 win against the Black Cats.
"He's a great prospect. He takes the ball and he runs at players. We know the ability he has got but you never know how they are going to react when they are thrown into games. There is a lot of pressure but he seems to thrive on it," Rooney told reporters.
"He's not shy - he's very confident, I think that's great to see. A lot of the young players who come through are a little bit timid and shy, but he looks comfortable and he's confident in his own ability, which you need to be."
Januzaj has nine months left to run on his current contract at Old Trafford, but United are believed to be keen to tie him down to extended terms amid reports that Juventus and Barcelona are keen on the 18-year-old's services.