Manchester United first-choice De Gea stepped in for Casillas in Friday's friendly defeat to France, sealed by a Loic Remy goal in Paris.
De Gea's outing at the Stade de France was his first start for Spain and just second appearance in total after a brief run-out against El Salvador in June.
The 23-year-old has long been touted as Spain's future first choice for years to come, but he is not letting his recent breakthrough get to his head.
When asked about his prospects of replacing the Real Madrid captain, De Gea told Cadena SER: "He's young for a keeper and still has plenty of good years left.
"We'll have to see what happens, it's the coach's call.
"I have to train as well as possible and give a good account of myself.
"I found out that I was going to start against France in the pre-match team talk. I was a little surprised, but also happy."
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