Di María signed for United for a club-record fee of €68m just after helping Real Madrid win their first Champions League title in 12 years and reaching the World Cup final with Argentina.
He has now recounted his nightmare at United under Van Gaal in "Ángel Di María: Breaking the Wall," a Netflix documentary about the Argentine's career that will be released soon.
He recalled, "We were winning, I was providing assists and scoring goals, but he didn't mention that. He only wanted to talk about mistakes. Identifying mistakes and learning from them is fundamental, of course.
"But given my position as a striker, it's inevitable that I make mistakes because I always try. My job is always to look for the assist or a shot on goal."
Former United teammate Ander Herrera confirme Di Maria comments: "Van Gaal was the kind of coach who very closely analyzed what his players were doing. And I was surprised that he thought it was unacceptable for Ángel to make three or four mistakes per game.
"There came a point when things got out of hand. Compared to the rest of the team, it seemed like the coach was much stricter with him. In the end, I think all the criticism ended up undermining Ángel's confidence."
Di Maria is now playing at home with Rosario Central in Argentina, where Herrera is also turning out for Boca Juniors.