The Spanish custodian has said that the entire squad want the striker to stay and labelled the Argentine as one of the best in the world in his position
The Argentine striker's future at the Santiago Bernabeu has been a hot topic of debate in recent weeks after a season in which he found himself behind Karim Benzema in Madrid's pecking order.
PSG playmaker Javier Pastore has publicly stated that his compatriot has been in talks with the French club, but the feeling inside Madrid's dressing room is that the 24-year-old is going nowhere.
"He is a crucial player for us and one of the top five strikers in the world, no doubt," Casillas is quoted as saying by AS.
"We are convinced that he will stay. He is happy here, but his future belongs to him.
"Jose Mourinho has said that if he was to leave, his release clause would have to be met. If that happens, there is nothing that we can do.
"He is a crucial piece in this team and we want him to stay."
As Madrid celebrate their first league crown since 2008, Casillas stated that he hopes the club will go further next season and win the Champions League crown.
"I want to win our 10th Champions League right away," he said.
"After a season in which we won La Liga, we have taken one step forward and I hope that next season will be better than this one."
Casillas has made over 450 club appearances in his 13-year career in the Spanish capital.