Januzaj left United last month to join Real Sociedad in a deal believed to be worth around £9m after falling out of favour at Old Trafford.
But now the Belgium international is hoping to prove his critics, including Van Gaal, wrong during his time in Spain.
“If I score a goal I will dedicate it to Van Gaal,” Januzaj told AS. “I don’t want to talk about him [Van Gaal] but everyone knows how many problems I had with him. We didn’t have the best of rapports. It wasn’t easy, it’s very frustrating and difficult when you are in a situation like that.”