Koeman harbours no regrets over his substitution choices after the Netherlands let their lead slip twice against Japan
Two expertly taken goals from captain Virgil van Dijk and Crysencio Summerville, either side of Keito Nakamura's strike, gave the Netherlands the lead on two occasions, but a deflected effort from Daichi Kamada two minutes from time meant the points were shared.
Six minutes after Summerville's 64th minute goal, Koeman substituted the winger as part of a triple change and later introduced a third centre back, Nathan Ake.
However, the more defensive setup backfired as Japan took control and Ogawa headed the ball goalwards, setting up Kamada's deflected equaliser. Koeman, though, stated he had no regrets regarding his tactics and blamed poor defending.
"There was also an issue with pressure on the flanks. Looking at the game, both goals, well, we did not defend well," he said to reporters.
"Football is a funny game because after Japan scored their second goal, they also began to defend. So we could have scored a third goal. I do not regret my choices."
A day after stating Memphis Depay was fit enough to start following recent thigh problems, Koeman brought the Netherlands' all-time leading goal scorer off the bench in the second half, replacing the energetic Donyell Malen. Depay received a yellow card for a harsh challenge and was unable to have an impact.
The Netherlands, who reached the quarter finals at the 2022 World Cup in Qatar, have almost a week off before they face Sweden in their second Group F match in Houston.
Koeman stated the Netherlands performed at a "minimum standard" against Japan and accused the media of underestimating their Asian opponents.
"Of course we can play better and we need to improve during the tournament," he said.
"This was certainly not our best performance. Of course, we would have preferred to win the first game. We expected to."