The Netherlands reached the FIFA World Cup semi-finals thanks to substitute Krul, who replaced regular gloveman Jasper Cillessen at the end of extra-time and saved two penalties as the European nation won 4-3 on penalties after a scoreless 120 minutes.
Van Gaal's decision stunned many after Cillessen - set to retain his place against Argentina - shut out the Costa Ricans during the 90 minutes and extra time in Salvador.
But the incoming Manchester United boss said Krul's introduction was part of the plan due to his superior penalty-saving abilities.
"We told Krul that he would be a possibility for the pens, we didn't tell Cillessen," Van Gaal said post-game. "We didn't want to ruin his preparation.
"There is no question about who will start next game, it will be Cillessen. But we felt Krul was the better choice here.
"We know Krul is the better penalty killer, a bigger reach too. We have practiced penalties, I believe when you're used to a certain shot you'll be more comfortable taking that shot."
Van Gaal added: "This is the best group I've worked with. In terms of team spirit, togetherness."
The Netherlands will face Argentina on Wednesday in a repeat of the 1978 final in Buenos Aires, which the South Americans won 3-1 after extra-time.
