Speaking ahead of a charity match in Madrid, Casillas told La Sexta television: "The coach decides which team he thinks is best for each match and you need to accept the decision.
"I'm not used to being a substitute, but above Iker Casillas or any player is the team. What I need to do is continue training and try to recover the confidence of the coach."
Casillas, who has won a host of titles with Madrid and captained Spain to the World Cup and back-to-back European Championships, added: "I feel good, but it is the coach who decides who plays. It will be a nice battle with Antonio. There is a healthy rivalry between us.
"Throughout the week I could sense that I wasn't going to play but it's not a setback. The important thing was that the team won in Malaga to cut the gap to Barca and Atletico.
"Mourinho didn't explain anything to me. He doesn't do it when I play, nor when I don't."