The 32-year-old goalkeeper, long-standing captain for both club and country, saw Diego Lopez surprisingly chosen ahead of him at home to Real Betis on Sunday, Ancelotti's competitive debut at the helm.
Casillas was dropped and then suffered a bone fracture in his hand which kept him sidelined for a number of weeks under Jose Mourinho last season. When he returned from injury, January transfer window signing Lopez remained first choice.
The fragmentation of support for Mourinho among the players, the fans and the media in his final months at the Bernabeu were in a large part put down to the decision to keep Casillas on the bench. Some felt it was personal rather than professional.
The arrival of the genial Ancelotti has seen an easing of those tensions in and around the club, and many expected Casillas to be restored to the number one jersey for the new campaign.
"I took this decision for today," Ancelotti said after Real squeaked a 2-1 win over Betis.
"It's a decision taken on small details. I made it for this game. We'll see what happens in the next one.
"I spoke to Casillas, he is very professional and he is keen to play."
Casillas has been undisputed first-choice keeper for club and country for more than a decade.
He was won two European Cups and five La Liga titles with Real, and captained the national team to their trio of consecutive titles at Euro 2008 and 2012, and at the World Cup finals in 2010.
Del Bosque has always stood by him, and took him to the Confederations Cup and Ecuador for last week's 2-0 friendly win, which may be one explanation for Ancelotti's decision.
Casillas played the first half in Guayaquil and Barcelona's Victor Valdes, hardly a poor alternative, the second, but a tougher choice may need to be made for Spain's upcoming World Cup qualifiers if the Madrid man is going to remain benched.
Spain top Group I by a point from France with three games left to play, and they travel to play third-placed Finland on Sept. 6, their last away fixture. The Finns held Spain to a 1-1 draw back in March.
Valdes said he was surprised by Ancelotti's decision at a news conference on Monday.
"I have seen Iker make saves no one thought possible," he said. "For me, he is the best but I shouldn't say any more because it isn't my team."
Lopez has performed well when called upon at Real, and is arguably better in the air and with his feet than Casillas, but the man he is replacing is a national institution.
Online polls run by Madrid-based sports dailies Marca and AS both showed a narrow majority of readers wanted to see Casillas restored to the number one slot.
Real's starting XI for next Monday's trip to play Granada in La Liga, a venue in which they suffered a 1-0 defeat last season, will be headline news in Spain and could have Del Bosque rethinking his plans for next year's World Cup finals in Brazil.
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