The Argentinian, who signed for a British transfer record fee of ??59.7million from Real Madrid last summer, has faded following a promising start.
Di Maria, 27, has not scored in the Premier League since the first week of October and on Saturday the playmaker put in possibly the worst performance of his career against Sunderland and was subsequently substituted at half-time.
As Van Gaal pointed out, Di Maria is not the only player to have struggled at United this year.
The Dutchman believes many of his summer signings have found it hard to hit the ground running and he believes they may all take until next year to acclimatise to top flight football.
“The first season is always difficult,” the United boss said.
“You can say it to every player we have bought: (Luke) Shaw, (Daley) Blind, (Ander) Herrera, (Radamel) Falcao and also Di Maria.
“For me it is not a big surprise, it is more a big surprise (if) he (had) adapted as quickly as possible.
“In the Netherlands, Spain and Germany I had the same experience with players.
“We have to give him time, not just for him but to all the other players.
“(The time frame) is dependable on the individual as there are so many different aspects of the game, of the match but also of the culture in the country.”