A disappointing World Cup placed pressure on the shoulders of the experienced stopper, but he did return to form for Real Madrid in the early part of the season.
More question marks have now been raised, however, after Casillas dropped a clanger in Thursday's 2-1 defeat to Slovakia.
"It's the custom to focus on one part of the match," Del Bosque told AS.
"What's going on with Iker isn't beyond repair or solution. There's too much focus on it. I don't think he's lost his form. He made a great save after 15 minutes.
"He's a fantastic player and all Spain fans should be proud of him."
Spain return to Euro 2016 qualification action against Luxembourg on Sunday.