The 18-year-old was not in the original squad list submitted by United to UEFA, but having sat out of the first two matches of the competition, the attacking midfielder has now qualified to feature as part of the 'B List'.
"Januzaj is available," Moyes told reporters. "He's an exciting young player and he's making good progress all the time.
"He's someone who, I'm sure, will play a big part in the future of Manchester United. He's an exceptional talent."
Januzaj has made six appearances this season, scoring two goals in the process.