The former Barcelona man has fallen down the pecking order at Old Trafford after a bust-up with manager Louis van Gaal.
The 33-year-old was on the verge of joining Besiktas over the summer, but pulled out at the last minute, due to the terms of an agreed deal being changed by the Turkish club.
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The Spaniard joined the Red Devils in January after his 20-year association with Barcelona came to an end.
Having returned from a serious ACL knee ligament injury, Valdes deputised for compatriot David de Gea last season, making two league appearances for the Red Devils.
Following a reported bust-up over the stopper's reluctance to play for a United youth team, Valdes has fallen out of favour at Old Trafford, and has been left out of the club's Champions League squad this season.
Casillas, who left Madrid to join FC Porto over the summer, believes Valdes' treatment is a prime example of how football can be ungrateful.
The Spain number one told Mundo Deportivo: "I want to send all my support and a lot of encouragement to Victor, this is proof that at times football is ungrateful. He deserves to find a solution soon because of the type of player he is.
"He deserves to find a solution soon because of the type of player he is. Had they told us a year-and-a-half ago that he would be in this situation, no one would have believed it."
OUT IN THE COLD
Valdes is currently the third choice goalkeeper at United, behind De Gea and Sergio Romero, and could even drop below Sam Johnstone, should the youngster remain at Old Trafford past the loan transfer window.
For a goalkeeper who has enjoyed so much success throughout his career to be sitting in the stands, is a crime within itself.
However, if it is the case that he refused to play for a youth team, something Valdes refutes, then Van Gaal is well within his rights to drop the goalkeeper.
The stopper has taken to Twitter over to summer, to prove he is still training hard and giving his all to Manchester United, but if the goalkeeper is looking for first-team football, he will not find it under Van Gaal.