Belgium international Fellaini endured a disappointing debut campaign at Old Trafford and there were rumours in pre-season that Van Gaal could cash in on the midfielder.
However, the former Everton man has impressed under the Dutchman and his displays led to a nomination for March's Premier League Player of the Month award.
And Van Gaal has been delighted with Fellaini's attitude.
"The first time that I met him he was late because of the World [Cup] like Robin van Persie and so on," he said. "But Robin van Persie was a player of mine [with Netherlands], that was different.
"Fellaini, I didn't know him. I did know him as a coach, that he is playing at Manchester United and before that Everton, that he's playing in the national team and was a regular player in the [starting] line-up.
"The first time I saw him was here in Manchester, he was not coming to the USA [on United's pre-season tour]. Of course I have heard a lot of stories about him, but from the first training session he did what I asked.
"When I said, okay, you have to do that in another way, he was open and he wants to perform. I am not surprised and he had, I think, an unlucky season because he was injured a lot of times, and ill a lot of times and because of that he didn't play so much but when he was fit, he has always played for me, more or less.
"That's also because I was looking for a balance in the team and he can given a lot of balance in the team but of course I can change him also.
"It is dependent on the qualities of the opponent also, but when he plays like he is playing now, I can't change him."
