The 18-year-old has been pleased with the impact he has made at the club this season, and thinks that the arrival of Mata has been welcomed by his teammates.
"I am quite pleased with my year, I have done a lot of work and have been integrating into the first team," he told the club's official website. "I have played games and I think there is more to come. I hope to win trophies at this club and be the best player.
"He is a great player and I think I am going to learn a lot from him. He is a technical player who is very clever. He scores goals and gives assists. We are very happy to have Mata at the club."
Januzaj has scored three goals in 17 appearances for the Red Devils this season.