The former Blancos defender has stressed that the goalkeeper will have to work for his place if he is to return to the starting XI.
The goalkeeper recently got the green light from the medical staff after a spell on the sidelines due to a hand injury but has had to settle for a spot on the bench so far due to the January signing's fine form.
"Iker has always been a starter and I wasn't expecting him to be injured. Neither was I expecting Diego Lopez to return to the club and perform so well," Roberto Carlos told Marca.
"Mourinho is his own man, he has his own way of doing things and you have to respect that. Now Iker will have to fight for a spot because nowhere in his contract does it say that he will be in the starting XI for the next 40 years."
Casillas' last match for Madrid was the 1-1 Copa del Rey draw against Valencia on January 23 when he fractured his hand.