Van Gaal's team were irresistible from the moment Red Devils striker Robin van Persie made it 1-1 with a stunning diving header just before the interval and ran riot with four unanswered goals in the second half in Salvador.
Van Gaal, who takes over at Old Trafford at the end of the tournament, cut an animated figure as the goals poured in but was eager to focus minds in his post-match press conference.
"I did not expect that it would go this way, it could have been 6-1 or even more, and we had a very happy feeling," he said.
"This is a nice start, but we have nothing. If we do not win our next game against Australia we have made no progress, but now we are obviously in a good position."
Van Gaal shared a high-five celebration with Van Persie after his first goal and United fans will be relishing the idea of the pair being reunited at club level next season.
"The pass from Daley Blind for Van Persie was fantastic, the finish was great. There was so much feeling. If you can make a goal in that way it's great...really fantastic. It was great that he (Van Persie) came to me. That is a sign of appreciation and that appreciation is mutual," Van Gaal added.
The 30-year-old hitman was eager to share in some mutual back-slapping with Van Gaal, giving him the lion's share of the credit for the result.
"This is incredible and inexplicable," Van Persie told Voetbal International. "For us, this is a dream come true and we have to enjoy it - everyone in Holland must too.
"This is because of the coach. He has prepared us great and he predicted how the game would go. It's incredible, because it was exactly as he and the staff predicted for us."
Spain coach Vicente del Bosque admitted to feeling physically sick by the nature of the defeat.
Switzerland upset the eventual winners in the opening game four years ago but the manner of this humbling could be harder to bounce back from.
"I feel sick, ill at ease, but with the courage a defeat must not sink us," said Del Bosque, reported by Marca.
"It is not a happy moment, but first of all I want to congratulate the Dutch team, who were superior to us. We have been overwhelmed in the second half.
"We are a team of good professionals and good lads, everyone hurts, so there was no accusatory glances and that's good for the team. There will be time to talk."
The veteran coach also declined to zero in on the performance of Iker Casillas, who was to blame for two of Holland's goals in a shaky performance.
Asked directly about his goalkeeper's display, Del Bosque said: "I insist that it is not the time to blame The defeats do not come through the performance of a player, but by the weakness of the team. We cannot blame anyone, least of all Iker ".
- Sports & Recreation
- Iker Casillas
- Van Gaal
- Robin van Persie