Former Real boss Jose Mourinho froze out Casillas, despite the club captain being fit in the second-half of the season.
"I put myself in his place," Del Bosque told Marca. "People know that I can't interfere in sporting matters. I shall never do that. But I have concerned myself with the unfairness towards a boy who is a son of Real Madrid. His behaviour has been irreproachable, both in goal and out of it.
"Dreadful things have been said about him. To attack Iker is to attack everything Real Madrid stands for. There are people who simply don't realise what he means to Real Madrid, but there are many, many more who do."