The Real Madrid legend has begun the campaign as backup to Diego Lopez in the Blancos goal, with new boss Carlo Ancelotti continuing the precedent set by Jose Mourinho.
Casillas has yet to feature in any of Madrid's three Liga fixtures so far this season, but Del Bosque says the 32-year-old is still very much the main man when it comes to the national team.
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"Casillas is captain of the side and an important part of our team."
The 149-capped Spain international has not played for Madrid since fracturing his hand at the start of this year.
But a lack of involvement for club has not jeopardised his place in the national team, and that looks set to remain the case with Del Bosque reiterating his faith in his skipper.
"I would never say he has to play at Madrid. I am in favour of Casillas, in favour of his history and his trajectory," he added.
"He is going through a bad time and needs the support of those who have respect and admiration.
"It's not a comfortable situation for anyone, but mostly it is uncomfortable for him. But having said that, I am not going against anyone, only going in favour of Casillas."
The reigning world and European champions face a World Cup qualifier away to Finland on Friday, before a friendly meeting with Chile in Geneva on Tuesday.
Pepe Reina and Victor Valdes are the other goalkeepers named in Del Bosque's 23-man Roja squad.