United had enjoyed an unbeaten pre-season campaign, but came unstuck in Van Gaal’s first competitive game as Gylfi Sigurdsson’s 72nd minute goal earned Swansea a first league win at Old Trafford.
Van Gaal pointed to nerves among his players for the below-par performance, which saw new captain Wayne Rooney score an acrobatic equaliser early in the second half after Ki Sung-Yueng had put Swansea head on 28 minutes.
"When you have so much preparation time and work so hard, it can't get worse than this,” said Van Gaal.
“We had great confidence and it got smashed down. But it's one game of many. You are not champions after one game.
“The question is if we can play as a team, if we can reach the right level. We couldn't do that today.
“Because of our wins in the US, expectation is so high. But I saw a lot of nervous players out there today, making wrong decisions. You have to cope with that pressure.”
United have been the quietest Premier League club in the transfer market this summer and Van Gaal is aware of the areas he needs to strengthen before the window closes in just over two weeks’ time.
He added: “I have thought about new players, I have done since the US tour. I know the positions we need. But you can only buy the players when you know what they are expected to do.”
Van Gaal started the match with the 3-5-2 formation he had deployed in pre-season before switching to a flat back four at the break.
“We can work with both systems,” said the Dutchman. “I changed it at half-time to the system they have always played at Manchester United, then I changed it back again later.
“They showed unbelievable commitment and worked hard, but just didn't reach the level.”
Robin van Persie missed the game as he continues to build his fitness after reaching the World Cup semi-finals with the Netherlands, and Van Gaal said: “Players have to be fit. That's all there is to it.
“I couldn't use Fellaini or Januzaj at the start because they came back late from the World Cup, RVP came back a week later than them. He is training with us now, and he is getting closer to being available for selection.”