The Spain boss says he is "obliged" to back the Real Madrid goalkeeper, who has found himself rooted to the bench in recent months
The 31-year-old sustained a hand injury in January, forcing the signing of Diego Lopez, who has since sealed his place the Blancos first team.
And the former Bernabeu boss believes Casillas' is a special case but says he will not be tempted to field another goalkeeper in the upcoming Confederations Cup.
"It is a special case, it is a difficult situation for him,” Del Bosque told Spanish television show El Hormiguero.
“It’s an awkward situation for him, but we are obliged to support him at all times. And he will have this support, he will be with us whether he’s on the bench or on the pitch, the most important thing is that he is with the Spanish national team,"
Del Bosque went on to say that he does not feel he has the right to comment on Jose Mourinho's situation at the club.
"I will not trivialise an issue that is not for me to discuss. We must be respectful of our colleagues and the clubs to which they belong."
Finally, the 62-year-old said he was eagerly anticipating Friday's Copa del Rey final between Madrid and Atletico at the Bernabeu.
"It's a final and party for all of Spanish football. It's almost the last game of the season and there's a desire to enjoy the game and present a good image of our football," he concluded.