The Spaniard, who joined the club on a free transfer earlier this year, was not named in United’s squad for the Champions League group stages on Wednesday, and has previously clashed with Van Gaal.
Valdes, who is yet to feature for the club in the 2015-16 campaign, was previously criticised by the Dutchman for refusing to play for the club’s Under-21 team, which led to a stinging response from the former Barcelona shot-stopper.
Despite his latest snub, Valdes has vowed to keep working hard to try and force his way into the reckoning at Old Trafford, and has thanked United’s fans for their support.
“No matter what happens. The only way I know is to work hard every day,” he wrote on Facebook.
“Thank you all for your support!”
Valdes was expected to leave United in the summer, but remained at the club despite links with Besiktas and Valencia.