Casillas came under criticism for his performances at the World Cup, with the Real Madrid stopper said to be under serious pressure to retain his place in the team.
Indeed, De Gea replaced Casillas in the XI for Thursday's clash with France, but he has insisted that he is not thinking about dethroning the veteran just yet.
"I do not think a lot about replacing Iker. He is still young for a goalkeeper and can still have plenty of years at the top level in football," he told Cadena Cope.
"It's getting a little weird the way the team's changed. Those players have carried Spain for many years and have been key to this team, but the young people pushing hard will do just as well."
Casillas is expected to return to the Spain XI for Monday's Euro 2016 qualifier against Macedonia.