The Roja captain was heavily criticised following his display in the world champions' 5-1 drubbing at the hands of Netherlands in their tournament opener – a performance Casillas himself described as the worst of his career.
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"I wouldn't put Iker on the bench," Mourinho told Yahoo.
"The position of goalkeeper is a very specific one, and I'm not one for changing it just because of a bad game.
"I only change things because of form, which shows the keeper isn't in the right condition, doesn't have confidence, stability, giving off the feeling it is better to change."
He added: "Casillas has done great things with Spain, he's a keeper who needs the confidence of his coach, and one bad game, for me, isn't sufficient enough a reason to change a goalkeeper."
Casillas is Spain’s all-time most-capped player with 155 appearances.