Van Gaal's team went in at half-time 2-0 up thanks to goals from Memphis Depay and Daley Blind, but the 71-year-old was not happy with the overall display in the first 45 minutes and warned that if it is repeated against higher quality opposition, they will be knocked out.
"I was very critical at half-time in my analysis despite that we were in the lead 2-0," said Van Gaal.
"I think they were fantastic goals and one of these goals was really a team goal. However, in the first half we suffered so much, we were dispossessed so often and it is not necessary.
"That is not acceptable in a World Cup if you play top countries. In the second half we made a much better job even though we scored fewer goals."
Van Gaal insisted afterward it was a performance that would give his players confidence ahead of the last eight and reiterated his claim that his players are good enough to leave Qatar as world champions.
"We are very, very pleased," he added.
"I have been talking for a year that we can become world champions, not that we will but we can. I am talking about team building and I derive pleasure from the group of players and performance and results."