The Argentine left United for PSG earlier this summer in a £44m deal just 12 months after joining for a British record £59m.
Di Maria started just 20 Premier League games for United, scoring four goals, but was constantly shifted around the pitch as the Dutchman searched for a successful combination.
The winger told ESPN Radio in Argentina: “At Manchester United I played very well but then got injured. Things did not work out and I also changed my position.
“The coach [Van Gaal] was the one who decided that and I ended up being more on the bench than in the team.
“It is difficult to adapt to Van Gaal because he points to his philosophy. Everyone takes what he wants and does.
“I started very well with him and then had a couple of clashes. There came a time when I did not have a good relationship with Van Gaal and that influences much in a player’s mind.
“The positive thing is that I made many friends, which is the best thing in football. Things with them were very good.”