Januzaj stole the show in his first start at the weekend, scoring both goals in a 2-1 victory over Sunderland at the Stadium of Light.
David Moyes promoted the 18-year-old to the first-team squad during United’s pre-season tour of the Far East and Australia.
After his impressive performance on Saturday, the club are reportedly keen to register Januzaj in their Champions League squad today, and are understood to be stepping up efforts to sign him on a long-term contract.
Januzaj, who is of Kosovan descent, was born in Belgium, and is eligible to play for a number of other countries including Albania, Turkey and England.
The midfielder signed for United at 16 from Anderlecht in Belgium, and his contract expires next June. There is already interest from a number of big clubs in Europe.
Manchester United do not want a repeat of the Paul Pogba fiasco two seasons ago.
The young French midfielder left United for Juventus, despite being promoted to the first team by Alex Ferguson.
United had wanted Pogba, who was then 19, to agree to an extended deal, but the financial demands made by his advisers were too high, and he eventually left for Juventus on a four-year contract reported worth £20,000 a week, with his agent earning a six-figure fee from the move.
Juventus are understood to be leading the chase to lure Januzaj from United, with Barcelona also interested.