The Oranje coach has explained that the Newcastle goalkeeper knew he would be called upon for spot kicks but his first choice shot-stopper had been left out of the loop.
The Dutch coach made the remarkable decision to substitute his first-choice goalkeeper with just seconds go to in extra time at Arena Fonte Nova.
However, it proved a masterstroke, with Krul saving two of Costa Rica's five spot kicks as Netherlands triumphed 4-3 on penalties to set up a semifinal showdown with Argentina.
PHOTOS: U.S. fan reactions | Beautiful people in Brazil
"We thought this through beforehand," Van Gaal told reporters in Salvador. "Every player in my squad has certain skills and qualities. They don't always overlap but all thought that Tim would be the most appropriate keeper to stop penalties because he has a long reach.
"But we said nothing to Jasper because we didn't want to be faced with this information before the match because that would be disappointing for him. But every goalkeeper has his qualities and we felt that Tim had a longer reach and a better track record stopping penalties.
"So, we discussed all of this with Tim and we had him study Costa Rica's penalties [in their last-16 shootout success over Greece]. You will have seen that he dived in the right direction time and time again.
"So, all of us are a little bit proud that this play has helped us through."
Van Gaal also explained that it had been difficult for him to keep one substitution in reserve, given a number of his players were struggling with fatigue in extra time.
"I had decided [to use Krul] before the 90 minutes had ended," he revealed. "I thought that we were playing well and had created lots of opportunities so I hadn't had have to make many subs, but there was a chance of penalties.
"I didn't feel Memphis [Depay] had played well so I brought in Jeremain [Lens] before the end of normal time. Of course, in extra time I took into account that I shouldn't use all my subs so I could still use Tim.
"But we had some injuries and guys like Robin [van Persie] and Georginio [Wijnaldum] were exhausted. Bringing on Klaas was also a risk as we went to four defenders. So it was a risk, a bit tricky.
"Fortunately, it worked out. Because if it hadn't, I would have taken the wrong decision!"
Van Gaal also hailed his players for the way in which they had kept their composure, despite having hit the woodwork three times and been frustrated by Costa Rica's alleged time wasting and persistent fouling.
"I believe that the players were more controlled than the technical staff," he joked. "My colleagues were jumping up and down all the time! But I felt that my players behaved marvelously - despite the delaying tactics; despite that the referee allowing all sorts of time wasting to go unpunished. And I'm not even talking about his decision on all the fouls,
"So, hats off to my players. It was frustrating for us and there's a law in football that if you miss so many chances, the opponent beats you. But I saw the determination in my players to progress. They know exactly when they have to take responsibility and that's the key. It's not about me, or anyone else. It's about them.
"When I compare them to other teams I've coached, they are the strongest in terms of professionalism and taking on responsibility."
The Netherlands next face Argentina in Sao Paulo. It was put to Van Gaal during his post-match press conference in Salvador that the Albiceleste will prove a more attack-minded adversary than Costa Rica - but the new Manchester United boss dismissed the notion that Lionel Messi and Co. always take the game to the opposition.
"I can't really give you my opinion of the second half as I only saw five minutes but I thought Belgium were the better side in the first half," Van Gaal said of the quarterfinal clash in Brasilia earlier in the day.
"Argentina are obviously an excellent team, with great individuals, such as Messi. But if people say that Argentina attack all the time, well, that was not the case in my opinion in the first half."
Follow Mark Doyle on