Despite coach Vicente Del Bosque stating his continued support for the 32-year-old shot-stopper, Casillas was placed in the 'back-up XI' in Spain's primary training session on Tuesday, losing out to Barcelona's Victor Valdes.
Casillas holds 147 national caps to his name, the most appearances in the history of Spain, but his hold on the number one spot for his country has slipped after being marginalized at Real Madrid.
Diego Lopez, who played 22 of Real Madrid's final 25 La Liga matches last season after outgoing head coach Jose Mourinho seemed to lose faith in Casillas, has started the first three games of the new campaign.
"It's hard for Casillas at Real Madrid, because they have another great goalkeeper in Diego Lopez," Spanish striker Alvaro Negredo said on Wednesday.
"I know Iker is great but the coach has to decide and whatever the decision you have to respect it.
"The competition within the squad is very tough but you give everything to play, and if you don't get in, you support the team."
Del Bosque has been vocal in his support for Casillas but has neither confirmed nor denied the Real Madrid skipper will line up in goals in Spain's World Cup qualifier versus Finland on Friday.
"Casillas? I'm worried about Finland," Del Bosque said.
"I have an idea of what I'm going to do.
"We'll see what kind of form each of them (Valdes and Casillas) is in and what frame of mind they are in."
"I would never say he has to play at Madrid. I am in favour of Casillas, in favour of his history and his trajectory.
"He is going through a bad time and needs the support of those who have respect and admiration.
"It's not a comfortable situation for anyone but mostly it is uncomfortable for him."
Spain's training XI was completed by Koke, Albiol, Ramos and Alba in defense; Silva, Iniesta and Xavi in midfield; and Navas, Soldado and Negredo up front.
Spain currently sit top of UEFA's World Cup Qualifying Group I, one point clear of France, while Finland are third, five points adrift of Del Bosque's team.
