The 33-year-old Casillas made a public apology after the humiliating defeat on Friday night in Spain's first game at the tournament. Del Bosque said the whole team must be held to account however.
"If a team loses it's not down to the performance of a single player," said Del Bosque when asked if he would drop Casillas for Wednesday's crucial Group B game against Chile at the Maracana stadium in Rio de Janeiro.
"Defeats happen as a result of weaknesses in the entire squad, but least of all because of the performance of Iker Casillas," added the normally low-key Del Bosque.
The Dutch claimed emphatic revenge in Salvador for their defeat to Spain in the 2010 World Cup final in South Africa. Robin van Persie and Arjen Robben both struck twice. Van Persie scored one of the goals of the tournament so far when he hurled himself at a lofted long ball to head over Casillas. That levelled the game.
The Dutch scored four second-half goals without reply, leaving Spain almost certainly needing to beat Chile and Australia to keep alive dreams of defending their title.
After the brilliant header, Van Persie again got the better of the Real Madrid goalkeeper when he won a sprint to the ball following a heavy touch from Casillas to claim his second goal.
The whole of Spain was left shellshocked by the defeat. Madrid sports daily Marca mourned "a historic catastrophe", dubbing the Netherlands' performance "a goal-scoring spree of Biblical proportions".
Casillas seeks forgiveness
Spain were "hammered", Del Bosque said after the match.
Spain conceded just two goals at the last World Cup and leaked only three in their qualifying campaign to reach Brazil, leaving Del Bosque at a loss to explain how they let in five this time.
"I don't find the words to explain it, Spain has never been a very defensive team, but we have always coped very well defensively. We were very weak against the Dutch," he said.
Casillas was devastated by the result. "I have to ask forgiveness for the match we played in general and me in particular," he said after. "I was not good enough."
But influential defender Sergio Ramos said Casillas would bounce back.
"The pressure and all the bad things fall even more on the players that have played the longest. I feel the same and we have to accept it.
"Iker isn't happy, just like the rest of us, but he is motivated and convinced that we can keep going in this tournament. When you lack that extra motivation or luck these things can happen, like in the second-half."
Manchester United striker Van Persie called the victory one of the greatest nights in Dutch football and said his header had been the best goal of his career.
Dutch coach Louis van Gaal believes the team can improve on the performance.
"That wasn't what you expect from your first World Cup match, I had expected the manner of the goals, but not that many," admitted the Dutch coach, who will link up with Van Persie at Old Trafford after the World Cup.
"It's everything to do with strategy and the willingness of the players to convert their chances with conviction."
Asked whether the Netherlands could improve, he insisted: "Yes, without a shadow of a doubt, you could see that in the last 20 minutes when we started to play the ball around.
"This result gives the players confidence and I expect us to play better."
The Dutch play their next Group B game against Australia on Wednesday in Porto Alegre.