The Real Madrid goalkeeper has insisted fitness and age didn't play a role in the reigning champions' shock World Cup exit
After losing their opening match 5-1 to Netherlands last week, Spain were beaten 2-0 by Chile on matchday two on Wednesday, meaning they can finish third in Group B at best behind their two conquerors.
"The reality is that we have several players who are in their 30s, but that should not be an excuse," Casillas told Telecinco.
"I don't know if these are my last games for Spain. I don't know. We'll see. It's up to the coach to make a decision and we will respect it. But this group of players cannot end like this.
"Does this signal the end of an era? Now is not the time to talk about this. Our opponents also play a role in what happened. We have to go through the difficult times as well.
"We are responsible for this. This hurts us more than anyone else. We can only congratulate Chile. They were the better team. There's nothing to say when you don't have luck and lose twice.
"We must look toward the future now and try to end the tournament with a win over Australia."
Spain meet Australia in their final group game on June 23.