The former Barcelona goalkeeper has recently recovered from a lengthy knee injury but has played just once for United’s reserves signing for the club as a free agent in the January transfer window.
The 33 year-old Spanish international finds himself behind compatriot David De Gea in the goalkeeping pecking order, with the latter having performed superbly for the Red Devils so far this season.
“When you are out for more than a year, first of all you have to show yourself at a lower level,” Van Gaal said.
Valdes won three Champions League and six La Liga titles at Barcelona and played his first United game in a 2-1 win over arch rivals Liverpool for the Under 21s in January, but he has been on the first-team bench on seven occasions.
Manchester United face Preston North End in the fifth round of the FA Cup on Monday evening, with the winner progressing to the quarter finals.