Mourinho, who worked with Casillas during his time at Real Madrid, believes that the player has proven in the past that he deserves to keep his place despite making a number of errors in Spain's 5-1 defeat to the Netherlands last Friday.
"I wouldn't put Iker on the bench," the Portuguese coach told Yahoo.
"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.
"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."
Cesc Fabregas has dismissed suggestions that his teammate's place in the team is under threat.