The Madrid glovesman was criticised after his performance in Sunday's Clasico versus Barcelona, but the former Atletico Madrid man has insisted that the 31-year-old remains the best shot-stopper around in the game.
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"Casillas is the best goalkeeper in the world, but he is also human. If you start to doubt Iker, then you doubt every single goalkeeper," De Gea told Radio Nacional de Espana.
"It's very difficult for a goalkeeper to win the Ballon d'Or. But as Iker Casillas is the best in the world in his position it would be nice if he won it.
"There are times when things don't go well and it seems as if you are a worse goalkeeper. But we are human."
De Gea then went on to voice his opinion that goalkeepers generally do not get the respect the deserve.
"Goalkeepers are always judged by everyone very quickly. We are not valued very highly, people should value us more, because if you don't have a goalkeeper, you can't play a game," he added.
Casillas is currently preparing for Spain's World Cup qualifier versus Belarus on Friday.