The QPR midfielder believes the Spain international is the best goalkeeper in the world, despite losing his place at the Santiago Bernabeu amid an alleged feud with Jose Mourinho
Casillas has spent most of this season on the bench with the former Sevilla goalkeeper Jose Mourinho's preferred choice between the posts since joining the club in January.
The Portuguese coach's decision to consistently leave the Spain international out of the team has drawn widespread criticism, most recently from defender Pepe, with Granero offering Casillas his backing.
"It's hard to judge from the outside," the 25-year-old said in an interview with AS.
"Obviously there is a conflict. I know Iker very well and he is the best goalkeeper in the world. He is the captain of the team and has lifted the World Cup and the European Championship twice.
"I cannot give the view from outside but Casillas is Casillas and that is indisputable. Just because he's not currently a permanent fixture in the team doesn't mean he can't be discussed as a symbol, as a symbol and as a goalkeeper. But the coach is professional and smart enough to choose between goalkeepers.
"I can't say Diego Lopez doesn't deserve his place."
Granero, who has scored once in 23 Premier League games this season as QPR succumbed to relegation, nevertheless stressed that he would be content for the beleaguered Mourinho to remain with los Merengues in spite of the rumoured unrest and trophy-less season.
"As a fan I would be happy for him to stay because I like to have a good coach," the former Getafe player added.
"But if he goes, another will come in. Luckily we have a president who works hard and well for Madrid. He has compiled the best squad in the world and that's going to stay there. Madrid will keep fighting for titles, for sure."