Casillas has gone through a tough season at the Bernabeu and has often been booed by his own fans.
Meanwhile, De Gea is widely considered to be one of the best keepers in the world right now having put in a number of sensational performances for Man United.
Casillas insisted he will be going nowhere in the summer and that he will retire at Madrid.
Speaking about his future, Casillas said: “Everytime out you feel a little more nervous, but I think that’s a good thing for me. I’m staying with Real Madrid next season. I don’t see myself leaving the club before the season starts. I’ve said it many times, I want to end my career here.
“I’ve been the subject of a lot of criticism over the past several years, but I’m a guy who likes a challenge. You could say I’m stubborn.”
The news will give United fans hope that De Gea may well end up staying at Old Trafford.
It is unlikely that the Spaniard will want to compete for a regular spot in the starting XI with Casillas, who enjoys a special status at Madrid due to his years of loyal service. At United, De Gea has the potential to become a legend and the club are also willing to make him one of their highest paid players.