The Spain manager has admitted he was surprised by the goalkeeper's lack of playing time with Real Madrid this season.
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, which was Jose Mourinho's final game in charge before departing for Chelsea.
“I find it hard to believe that Iker dropped out of the squad,” del Bosque 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.
“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,” del Bosque said.
Casillas, Spain's captain, has been named in La Furia Roja’s squad for the upcoming friendlies against Haiti and Ireland, as well as the FIFA Confederations Cup later this month.