The Real Madrid goalkeeper says he is motivated to work hard in order to show his boss that his decision to drop him for their last two Liga games was a foolish one
The 31-year old came off the bench on Sunday to replace his deputy, Antonio Adan, who had been sent off after just five minutes for a clumsy foul.
He subsequently made several key saves in the 4-0 win over Celta Vigo in the Copa del Rey on Wednesday, and wants to prove his worth.
"I'm not going through a bad spell, but I have to train more and better and show that I deserve to be in the team. I have to show Mourinho that he made a mistake," he told reporters at a publicity event.
"I don't mind him saying that I was too comfortable in the team, because he is in charge of the team, and it's what he believes."
"He didn't give me an explanation when he dropped me, but I don't think he has to say anything to me. It was a decision he took and that must be respected."
Real Madrid find themselves well adrift of a rampant Barcelona in La Liga, and Casillas offered his insight on the situation.
"It's a team game, not for individuals. Last season it was very hard work to win the league, but well worth it," he continued.
"Certainly we have had some bad games, and we know it is practically impossible to win the league, but we must continue fighting as much as possible."