Valdes' exclusion from United's squad for their pre-season tour to the United States came as a surprise, particularly with first choice keeper David De Gea expected to join Real Madrid. Anders Lindegaard and Sam Johnstone were selected ahead of Valdes.
Van Gaal has now explained that the former Barcelona shot-stopper is no longer in his plans due to a difference in philosophy.
"He is not selected because he does not follow my philosophy. Then there is not one place for one player [who does that]. The philosophy is how you play football and how you maintain your match rhythm," Van Gaal said at a press conference in Seattle.
"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 others for how you have to play [if you are to be] a goalkeeper for Manchester United. When you are not willing to follow the principles of the philosophy, there is only one way and that is out."
Valdes, who had previously worked with Van Gaal at Barcelona, initially joined United to train as he recovered from a serious knee injury, but he was later handed a contract by the Old Trafford outfit. Van Gaal conceded that he felt let down by Valdes.
"For us it is a big disappointment," he said. "We thought he would follow our philosophy but it is a pity because we have given an opportunity to rehabilitate and then we gave a contract. Then when he is fit to play and when you are not playing in the first selection you have to play in the second selection. He refused that."
The Dutchman's comments come just days after Valdes took to Twitter to express his optimism ahead of the season, despite being left out of the touring squad.
After a Hard Training Session this morning, i am working hard for this Season ! Come on You Reds !!!! pic.twitter.com/eVStvlcPMj
— Victor Valdes (@1victorvaldes) July 14, 2015
It is not yet clear where Van Gaal will turn to boost his goalkeeping stocks, with the tactician admitting that the purchase of another experienced keeper is "dependent on David De Gea".