The Blancos legend says the goalkeeper has a tough competitor in Diego Lopez, and backs his fellow countryman Neymar to become a success at the Camp Nou.
The 32-year-old Casillas has not made a single appearance in La Liga this season and was injured a few minutes into his first game in eight months against Galatasaray last week. Ronaldo believes opting for Lopez instead of Casillas is not a hard choice to make for the coach, considering the shot-stoppers' respective quality.
"Casillas is in a difficult situation," Ronaldo told AS. "He has not had a competitor like this to date, but he is one of the best goalkeepers in history.
"I had Diego also training with me, and he is very good. So [coach Carlo] Ancelotti does not have any trouble choosing."
Ronaldo went on to back compatriot Neymar to shine at Barcelona, but believes he will need time to adapt to life in La Liga.
"I believe that good players always understand each other. Neymar will have difficulties at first but then do well playing in Europe with great players," Ronaldo said. "He spent his last year in Brazil without evolving. Though football is harder in Europe and the competition will make him better."
Finally, Ronaldo has voiced his belief that Madrid, Barca, Borussia Dortmund and Bayern Munich -- last season's semifinalists -- will battle it out for the Champions League crown this season.
"Madrid have a good chance of winning it, but also the Germans, who played very well last year," Ronaldo said. "Madrid and Barca are always favorites. I believe it will be between the Germans, Barca and Madrid."