The goalkeeper has revealed in a biography about the current Spain boss that he was "really affected" by the refusal to offer the coach a new contract back in 2003
The current Spain boss spent four years in charge of the capital club between 1999 and 2003, during which time Los Blancos won two Liga titles, two Champions Leagues and the Club World Cup.
However, the club ultimately decided against offering Del Bosque a new contract, much to the shock of goalkeeper Casillas.
“His departure from Real Madrid really affected me," the current Blancos captain writes in the preface of a new authorised biography on Del Bosque. "It was as though some part of Real Madrid had gone too ...
"On the night it happened, I was on my way to an event in Alicante when I heard that Vicente was leaving.
"We had an inkling that something was going to happen [but] the celebrations for that league [title win in 2003] were the worst I can remember”.
Casillas also revealed that Del Bosque played a key role in his development as a player and as a man, by dropping him from the Madrid starting line-up back in 2002.
“He helped me but he also made me aware of the reality of football, of sport and life," the World Cup winner mused. "He gave me confidence – and then a good dose of what it means to be on the bench.
"At the time, it made me angry because it’s hard for any player to suddenly find themselves out of the team.
"After going over it in my head, I started to understand and when I was presented with an opportunity, he showed that he still had faith in me – like when he brought me on in the Champions League final in Glasgow [in 2002]."