Manchester United manager Louis van Gaal said Wednesday goalkeeper Victor Valdes is to be sold after "refusing to play" for the club's second team.

Van Gaal said former Barcelona keeper Valdes, who only signed for United in January, faced an early end to his Old Trafford career after failing to follow the club's philosophy.

"When you are not willing to follow the principles of the philosophy, there is only one way, and that is out," van Gaal said at a press conference to unveil United's new signings at the start of their US tour.

Valdes, whose signing in January was widely seen as a pre-emptive move ahead of the likely departure of first choice goalkeeper David De Gea, was surprisingly not included in the squad which arrived in Seattle on Monday.

Van Gaal revealed the 33-year-old Spanish international had not travelled because "he does not follow my philosophy."

"The philosophy is how you play football and how you maintain your match rhythm," the Dutchman explained.

"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 like a goalkeeper for Manchester United."

Asked if that meant United planned to sell Valdes, van Gaal replied: "Yes."

Valdes was invited to train with United in October 2014 as he sought to work his way back to fitness from a knee injury.

He signed in January and made his first start in the final game of the season against Hull, keeping a clean sheet.

Van Gaal admitted he was dismayed that Valdes's fleeting career at the Premier League giants was coming to a premature end.

"For us it is a big disappointment," he told reporters.

"We thought he will follow our philosophy but it is a pity because we have given an opportunity to rehabilitate and then we gave a contract and 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."

Van Gaal suggested he had made the decision to sell Valdes before the end of the season, claiming the goalkeeper had only been selected to face Hull in May to put him in the shop window.

"I want to help him and that he can show his qualities for another club," van Gaal said. "I am always a very social human being."

United open their International Champions Cup campaign against Mexico's Club America in Seattle on Friday.