Spain coach Vicente del Bosque said he had still not decided whether Casillas, Victor Valdes or Pepe Reyna would face Uruguay in Sunday's Group B match, though he added that whoever it was would keep their place for the following two games.
"It is more difficult because of the last four months, waiting for the line up is hard," Casillas told reporters
"I have become used to not being a regular first choice at my club but the fact that I was called up to the national team made me aware that my team mates were happy with me."
"I've tried to train and take advantage of the opportunity to show my team mates and coach that I am in form and prepared."
Del Bosque appeared sympathetic.
"He has just been through what was an unusual situation for him over the last months," he said. "It has not been easy for him to adjust so it is not a surprise that he feels a bit nervous."
"The same goalkeeper that plays tomorrow will play in the first three games of the first round."
Casillas, at 32 still relatively young for a keeper, is hoping to add to his Spanish-record 145 caps at the warm-up tournament for next year's World Cup.
He came on for Barcelona's Victor Valdes with just over half an hour left in Tuesday's friendly against Ireland and conjured up two trademark reflex saves to deny the Irish an equaliser. Spain went on to win 2-0.
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- Victor Valdes