Roy, who is now head of Ajax's reserve side, believes that Van Gaal would be a perfect fit for the English giants - who sacked David Moyes on Tuesday.
The former Dutch international and Nottingham Forest forward played for Van Gaal during their time together at Ajax.
"He has definitely still got the hunger. I think he will die on the pitch, because he will never retire. He loves the job too much," he is quoted by the Daily Mirror.
"He will never turn down Manchester United, believe me. I think it's the job he really wants. And he will be a revelation at Manchester United.
"He is, without question, the best manager I have ever played for. For me, he has proved himself to be one of the best managers and coaches in the world.
"He is very disciplined, very strict and is very attacking in his philosophy. I know English football well, and his style is suited to that.
"He believes in wingers, he is very strong and very tactical. The best word I can think of is demanding.
"On the pitch, every player will be told their job, they will know exactly what is expected of them, their position and role in the team.
"The way he prepares his teams is outstanding. In terms of organisation and tactics - the way he sets teams up will suit English football - is superb.
"I don't know so much about fitness anymore and that's the only thing he may lack, simply because you have to be so fit to play in England. That's such a big part of the game in England.
"But if you look at the teams he has managed, they have always played attractive and exciting football. He will bring that to Manchester United. If he goes there, he will be a huge success."
Roy believes than Van Gaal would help United's youngsters flourish, as well as fellow countryman Robin van Persie.
"I look at Manchester United and they have a lot of young players," he said.
"They will respond to him as well as the older ones who will have respect for him.
"I know Robin van Persie will work well with him. He can handle the big clubs because of where he's been before and the big players.
"He can be difficult and he is strong and strict. I can't deny that I had some problems with him but I was a young player and you learn. We go back to the word demanding and he always demands his players to know their job, to work hard and do their best.
"You must do that because of the way he sets up his teams. Everyone must give 100 per cent to make his style work. If you give him everything then he is a very good manager for the players and they will learn and be successful under him."