The goalkeeper has endured a difficult couple of seasons after losing out to Diego Lopez as the club's first-choice goalkeeper and a poor World Cup showing that saw Spain eliminated at the group stage.
But despite his recent form, the sale of Lopez to AC Milan means Casillas is likely to be reinstated as the main goalkeeper ahead of new signing Keylor Navas and Ronaldo says the team will be in safe hands with the 33-year-old, who has three Champions League titles, two European Championships and a World Cup to his name.
"Iker has never stopped being one of the best keepers in the world," Ronaldo told reporters. "He has always been world-class."
Madrid kicked off their season with a Uefa Super Cup win over Sevilla on Tuesday, with Ronaldo scoring both goals in a 2-0 victory, and the Portuguese forward insists the trophy is just the beginning for reigning the European champions.
"We have to think about the now, not the future," he added.
"First one objective and then another. We've won the European Super Cup and that's great, but we have to keep on working. We've started the season off on the right foot and we're pleased with that."