The world and European champions were beaten 2-0 by Chile on Wednesday night to follow on from their 5-1 humiliation by the Netherlands last week, and the long-serving goalkeeper does not know what the future holds for many of La Roja's squad, including himself.
A dejected Casillas told AS: "Could this be my last game? I don't know, I really don't know.
"We have a few players who are over 30; it's just a shame that this generation had to bow out [of the World Cup] in this way".
Spain still have to play Australia in their final group game.