Although Atletico Madrid moved quickly to quash the claims about their prized asset, stating that a deal has yet to be completed, Casillas believes that a move to the Premier League will only enhance De Gea’s talents.
"He will soon be fighting for my place. He's a great keeper,” Casillas said in The Sun.
"We have to take care of him because he is the future. He is 20-years-old. He will pension us all off.
"I hope he does great things for his club and the Spanish national team."
De Gea rose to prominence as an 18-year-old after Atletico suffered an injury crisis in goal, with the stopper going on to cement his place in the first-team, as well as becoming a regular fixture in the Spanish under-21 side.
"He will learn a lot from the fact he has started so young. I had it as well,” added Casillas.
"The people of Atletico Madrid have to be so proud that such a great keeper has come out of their academy.
"Everyone has seen what he's been doing and praising his performances. Watching him reminds me a lot of when I started 12 years ago.
"Between us all, we have to look after him. He has a long road in front of him and when he least expects it he will get a call-up to the national team.
"I'm not suddenly discovering De Gea now everyone has seen what he is capable of doing. He's a good professional and a great person and that for me is very important."
De Gea was quick to return the praise to his compatriot, saying that Casillas is a role model and the best goalkeeper in the world.
"I think not just for me but for many keepers, he's a point of reference and an example to follow," he said.
"He has incredible qualities and right now he's the best keeper in the world."
