Januzaj was sold by United this summer to La Real.
"There was not the same freedom," he told the Daily Mail of Van Gaal's management. "A winger needs that confidence, a manager telling you, 'Go and take people on'. With him, it was a slow passing game.
"For some it was difficult. You could see some players were not enjoying the football. The manager and I had many difficult meetings. I was frustrated and everyone could see that I was not the same Adnan as the first year. I was sometimes even in the stands. When you work hard in training and then get left out, it's hard for a young guy.
"As a footballer, if you think too much on the pitch, it is never good. You need instinct in your play. When the ball came to us, we had to stop and think, "What are we going to do with it here?", 'I can't lose the ball'."