Di Maria arrived at Old Trafford in August among much fanfare in a British-record £59.7 million transfer from Real Madrid.
However, the Argentina international has flattered to deceive at times this season and has managed just three goals in 19 Premier League appearances.
Di Maria was hauled off at half-time of Saturday's 2-0 victory over Sunderland, but manager Van Gaal stated that moving to a new team has been a learning curve for all his signings - not just Di Maria.
"I think it counts for all the players that we have bought in the first season it is always difficult," he said.
"I have explained that in the press conference after the match [against Sunderland] and you can see it through every player we have bought - [Luke] Shaw, [Daley] Blind, [Ander] Herrera, [Radamel] Falcao, also Di Maria.
"For me it's not a big surprise. It's more a big surprise when he adapts as quickly as possible. [I experienced it] in the Netherlands, Spain, Germany.
"I had the same experience with players so we have to give him time. I have already said not just about Di Maria but to all the players.
"It [settling] depends on the individual. You have to manage all the aspects, the culture in the countries. The general answer is one year."
Van Gaal also confirmed that midfielder Michael Carrick is not fit to start Wednesday's top-flight clash against Newcastle United at St James' Park.
The England international returned from a month-long absence to take his place on the bench against Sunderland, but Van Gaal does not want to take any risks with his fitness.
"I think it is too early for Carrick," he added. "He trained three sessions with us I think because of the players I didn't bring with us, he was in the 18.
"As a sub you can play 20 minutes, that he can do."
