The Dutchman was speaking after United's humiliating 4-0 loss at third-tier Milton Keynes Dons in the League Cup second round had compounded their terrible start to the campaign.
"I think that he (Di Maria) can contribute and that is very important but we don't have to expect that from tomorrow our world has changed," van Gaal told Sky Sports after the Argentina international signed for 59.7 million pounds ($98.77 million).
"He has to adapt to the culture of England, the culture of the Premier League and the philosophy of Manchester United.
"He shall contribute very much in our way of playing. He is a creative player. I can change the system with Di Maria. He can play as a winger and he can also play in midfield," he added.
The experienced Van Gaal has yet to fashion a win during his three competitive matches in charge of United this season.
Their thrashing by MK Dons followed a 2-1 defeat against Swansea City, United's first opening day loss at home since 1972, and a 1-1 draw against Sunderland in the Premier League.
Van Gaal praised the reaction he got from the Old Trafford faithful during the reverse to Swansea and urged them to remain patient as he looks to reshape his struggling team.
"(The fans) have to believe in our philosophy," he said. "We are building up a team that you cannot make in one month or even one year.
"It (Tuesday's defeat) is very disappointing but I hope they shall maintain their confidence in the club, and in our philosophy because that philosophy takes time.
"We know exactly what we are doing. We have lost in the Capital One Cup, and so tonight has cost us the chance of a trophy, and that is disappointing."
United's next game is a trip to promoted Burnley in the Premier League on Saturday.