The coach was unhappy with his side's defending and lack of fighting spirit as the Oranje surrendered a two-goal lead to Japan in the second half of Saturday's friendly
Rafael van der Vaart and Arjen Robben struck first to give the Oranje the advantage, but their goals were cancelled out by Yuya Osaka and Keisuke Honda either side of half-time.
Makeshift centre-back pairing Ron Vlaar and Stefan de Vrij did little to impress during the 90 minutes but Van Gaal insisted his entire side were to blame for their drop off in performance in the second half.
"The decline was much too big. This should never happen to us," the coach told reporters after the match.
"The team defended poorly, therefore the centre-back pairing made mistakes. This makes sense. During the second half we were not the better team. The long ball is not our game. We lost every second ball.
"If we lead 2-0 in the first half, you know you can play the same way after the break. We let Japan back into the game, and that was completely unnecessary."
Netherlands host Colombia in Amsterdam on Tuesday in their next friendly encounter, with the South Americans full of confidence following their 2-0 win over Belgium on Thursday.