La Roja opened the scoring in the World Cup Group B clash, but conceded four second-half goals as Netherlands thumped the holders.
"It's the hardest loss of my sporting life. The positive news is that we're still in the tournament, and nothing is lost," he told reporters.
"It was a very bad game. A debacle. We did everything badly. Their goal hurt us. We were all bad. We need to have a look at ourselves. We can't wait to put things right on Wednesday.
"We want to show this side can improve, and that we can do things at this World Cup. It's hard because we messed up big time."
Spain return to action against Chile on Wednesday.