Coach Vicente del Bosque is spoilt for choice with Real Madrid's Casillas, Liverpool's Pepe Reina and Barcelona's Valdes vying for the number one jersey.
Valdes has never won a cap for the national team despite having won La Liga's award for having conceded the fewest goals in 2009 and 2010, and there have been some calls to promote him above long-time regulars Casillas and Reina.
"Iker has an impeccable record. He's achieved a lot and I'm sure he will win more titles," Valdes told a news conference. "I can't believe that people would doubt his abilities."
Spain captain Casillas, 29, was at fault for Saudi Arabia's opening goal on Saturday, when he failed to get his fist to a punch at a corner giving Osama Hawsawi a free header.
The European champions battled back to win 3-2 in a game where Casillas won his 103rd cap to become Spain's second most capped player behind another keeper Andoni Zubizarreta, who made 126 appearances.
Valdes may get the chance to win his first cap in the friendly against South Korea in Innsbruck on Thursday, the other two debutants in the squad Pedro and Javi Martinez having broken their ducks against the Saudis.
"The coach hasn't decided the order for the goalkeepers but I'm well aware that I was the last one in," Valdes said.
"I'm lucky to have two colleagues like Casillas and Reina to work with and to help me improve."
Striker David Villa refused to be drawn into choosing between the three.
"We're aware that any one of them would be first choice in 90 percent of other national sides. It's difficult to avoid the debate," he said.
"It's a luxury ... but it is the coach who must decide."
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