The Belgian proved to be the match-winner at the Stadium of Light as the 18-year-old scored two second-half goals to help secure a 2-1 victory over Sunderland.
Januzaj admits he is learning “little things” in first-team training from more experienced Red Devils stars, like Shinji Kagawa, Rooney and Van Persie.
“Everything is quicker,” Januzaj told United’s website.
“The games are quicker, you have to think faster and play quicker. I’m training with great players almost every day so I do what they tell me but at the same time I want to bring my own qualities too. That’s important. You have to do both.
“The technical qualities [of the likes of Robin van Persie, Shinji Kagawa and Wayne Rooney] are brilliant, so just playing alongside them raises your standards.
“You learn little things like when to speed up play, when to pass, when to hold the ball, all those things… by watching experienced players it just becomes natural.”
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