The Spain skipper admits he feels at a disadvantage to Pepe Reina and Victor Valdes having been out of regular action since the turn of the year.
The Roja captain played the first 45 minutes for his country in the 2-1 win over Haiti on Saturday and enjoyed a quiet first half in goal, before being replaced by Liverpool's Reina at the interval.
Having been out of action since January following a hand injury and a falling-out with former Real Madrid boss Jose Mourinho, Casillas admits he faces a battle to keep his place for the up-coming Confederations Cups.
"I've been out about four months," the 32-year-old explained to reporters after the match. "After so long without playing I was both happy and nervous tonight.
"I see myself fighting from a spot from the start with a disadvantage to my peers, going up against Pepe and Victor. Not playing for long puts a dent in a player like me.
"Sometimes it's good to go through that, to appreciate it more when you play."
Casillas went on to play down suggestions of a spat with club-mate Alvaro Arbeloa after the defender backed former boss Mourinho in recent weeks, insisting the matter is water under the bridge for the time being.
"We have finished the season with Madrid, and now it's a new period with the Spain squad. Alvaro has his point of view and that must be respected.
"I don't have to talk further about him. The important thing is that I've returned to playing."