The 19-year-old broke into the senior side under David Moyes's management last term, but so far with Van Gaal at the helm, he is yet to start an encounter.
To gain match fitness, he was included in United's Under-21 side against Sunderland last night and responded by scoring a hat-trick in a 4-0 victory at Old Trafford.
Speaking after the match, the Belgium international told the club's official website: "I tried to do my best. Obviously, even if it's reserves level, I always have to show myself and improve. I got a hat-trick but I think I worked hard defensively as well and these are the kind of things I have to do now and at senior level.
"Of course, I played a lot of games last season and I'm trying to impress the manager once again and get games again this year as well."
Januzaj made 35 appearances in all competitions last season.