Krul was brought on to replace regular keeper Jasper Cillessen just as extra time was ending with the scores locked at 0-0. He saved two spot-kicks in the ensuing shootout and the Netherlands edged past Costa Rica 4-3.
"That didn't occur to me," Van Gaal told reporters on Tuesday, when asked whether he had thought about starting Krul against Argentina.
"This is something I excluded from the very beginning, because I believe Jasper Cillessen is doing a wonderful job.
"Stopping a penalty, that is what the other two are better at, so I selected Krul."
The Netherlands' other reserve keeper is Michel Vorm.
The unorthodox switch has been one of the main talking points of the latter rounds of the World Cup, particularly because it succeeded so spectacularly.
Cillessen had been kept in the dark about Van Gaal's plan, and initially looked upset when he was substituted.
Van Gaal was not asked at the news conference whether he would choose Krul again should the semi-final go to penalties.
