The Netherlands boss, who will leave his post with the Dutch FA after this summer's World Cup, was officially confirmed as David Moyes's successor on Monday.
Januzaj burst into the first team under Moyes last season, but the 19-year-old has expressed his delight at Van Gaal's arrival.
"I feel very happy because he's a great manager," Januzaj told Sky Sports News. "He's won a lot and I'm looking forward to working with him. I think we're going to have a great future ahead, so I'm looking forward to it."