Di Maria, signed from Real Madrid this week for a British record 59.7 million pounds ($98.96 million), told a news conference at Old Trafford that he had joined the 20-times English champions to help them "become an important team again".
United manager Van Gaal told reporters: "I'm very pleased that we can have such a class player in our squad. But we have many class players and for me it's important he is a team player. That is what I am liking in Di Maria.
"Not only class, he can accelerate the game for us and can also play in the interest of the team.
"That's why we have chosen this player and I hope he shall give us good results, but we cannot expect after two days' training sessions that he is the miracle. No, you have to work for the miracle, and he knows that."
The Dutchman added: "I have said that I only buy players that can improve our philosophy. I have spoken already about his way of playing in the interests of the team that I like.
"He fits in our philosophy and I can change the system with him and maybe we will do that in the future, maybe in a match. At Real Madrid he didn't only play as a winger but also in midfield, so that is also a reason why we are convinced he shall succeed."
Van Gaal and United need a reversal in fortunes after a wretched start to the season. They have picked up one point from the opening two Premier League games and, to the amusement of some and the surprise of many, were humbled 4-0 by third-tier Milton Keynes Dons in the League Cup this week.
Di Maria, 26, said the lack of Champions League soccer at Old Trafford this season had not put him off leaving Real for United.
"Manchester United have already won many trophies and are a very big team," he said.
"I have come to help raise the game and become an important team once again. I want to win many trophies with them.
"I'm very pleased to play for Manchester United. I like the team and I wanted to come here even though they're not playing in the Champions League. I want to help the team get into the Champions League."
He said former Real team mate Cristiano Ronaldo had been instrumental in his choosing to wear United's No 7 jersey, the number worn by previous greats including the Portuguese winger, David Beckham, Eric Cantona, Bryan Robson and George Best.
"Cristiano spoke to me at Madrid and explained how important it is," Di Maria said. "I hope I can live up to it."
Di Maria is expected to make his debut against Burnley at Turf Moor on Saturday.
Van Gaal suggested he hoped to add further new signings before the transfer window closes on Monday.
"The club will (announce) when the time's there," he said. "Players will leave, others will come."
As well as Di Maria, United have signed Ander Herrera, Luke Shaw and defender Marcos Rojo in the close-season. Shaw and Herrera are now injured, however.
"We have 10 injuries - one more than last week," Van Gaal said ruefully.