The 62-year-old has admitted he has been surprised by the goalkeeper's lack of playing time at club level
The Spain coach praised the goalkeeper's attitude during his time working with the 32-year-old, who made just 19 Liga appearances in 2012/13 and was left out of the squad completely for los Blancos' final game of the campaign against Osasuna.
“I find it hard to believe that Iker dropped out of the squad,” he told reporters.
“It’s hard to believe because I’ve known him for many years and his career has always been impeccable."
Del Bosque has not spoken to Casillas about his lack of playing time and insisted it was a matter for Madrid to deal with.
He added: “I have not had the chance to speak to him since the game we played in France, but while it’s none of my business and an internal matter for the club I have no reason to believe it is true.”
Casillas has been named in Spain’s squad for their upcoming friendlies against Haiti and Ireland, as well as the Confederations Cup later this month.