Valdes, who was back-up to compatriot David de Gea at United last year, did not travel with the rest of the squad to America for the club’s pre-season tour, but no reason had previously been given for his exclusion.
However, at a press conference in Seattle on Wednesday to present new signings Bastian Schweinsteiger, Matteo Darmian and Morgan Schneiderlin, Red Devils boss Van Gaal revealed the reason behind Valdes’ non selection.
“He is not selected because he doesn’t follow our philosophy and there is no place for that one player,” Van Gaal said.
“Philosophy is how you play football and how you maintain your match rhythm. For example, he refused last year to play in the second team.
“That is one aspect of our philosophy… but there are a lot of other aspects in the philosophy of how you have to play like a goalkeeper at Manchester United.
“When you are not willing to follow the principles of the philosophy, then there is only one way and that is out.
“It is a big disappointment. We thought that he would follow our philosophy. It is a pity because we have given him the opportunity to rehabilitate, gave him a contract and then he is fit to play.
“If you do not play in the first team, our philosophy is to play in the second team and he did not do that.”