Louis van Gaal has strongly suggested Victor Valdes' time is up at Manchester United after the Dutch coach branded him unprofessional for refusing to play for their reserves last season.

Van Gaal was severely critical of the World Cup winner for failing to follow his philosophy, and said he did not act in the way a United player should.

The goalkeeper was a surprise omission from United's tour party for the preseason trip to the United States, when David De Gea, Anders Lindegaard and Sam Johnstone were selected instead.

And Van Gaal bluntly said that Valdes has no future at Old Trafford, even as he admitted he is unsure if Real Madrid target De Gea will stay with United.

Van Gaal revealed in a news conference in Seattle: "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.

"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."

However, Valdes hit back on Twitter on Thursday by posting a message that simply said "...? #respect" and contained four examples of him playing for the U21s last season.

Van Gaal first coached the Spaniard at Barcelona and they were reunited when Valdes joined United, initially to train and then on a contract.

And he said he feels the 33-year-old has let him down, saying: "For us it is a big disappointment. 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."

Valdes made two appearances for United at the end of last season with his sole start coming on the final day against Hull.

And asked why he was selected after refusing to play for United's under-21s, Van Gaal explained: "I want to help him and that he can show his qualities for another club. I am always a very social human being."

Victor Valdes has been a 'big disappointment' to coach Louis van Gaal. Martin Rickett / PA Wire/Press Association Images

Van Gaal, who has been linked with Argentina international Sergio Romero, has admitted he may need to sign another goalkeeper and suggested it is up to De Gea if he joins Real Madrid.

"It is dependent on David De Gea," he said. "Shall he stay? He is training here and doing his utmost best."

United are holding out for world-record fee for a goalkeeper, topping the £32.6 million Juventus paid for Gianluigi Buffon in 2001, and have vowed to hold De Gea to the last year of his contract if they do not receive one.