Casillas has long been a mainstay in the Real goal in a trophy-laden 15-year career at the Santiago Bernabeu.
After picking up five league winners' medals, Casillas found him usurped as the number one choice in La Liga by Diego Lopez under Jose Mourinho and then Carlo Ancelotti.
Ancelotti opted to select Casillas for Copa del Rey and UEFA Champions League matches during 2013-14 - both competitions that Real won.
However, the goalkeeper's lack of regular playing time led to signs of rustiness, and the 33-year-old was at fault for Diego Godin's goal in the Champions League final win over Atletico Madrid in May.
Casillas followed that mistake up with costly errors in Brazil as Spain's World Cup defence ended in group-stage elimination.
With Lopez now departed for Milan, Casillas is expected to battle with new arrival Keylor Navas for a regular starting spot, but Ronaldo has dismissed any talk that his colleague is past his best.
"Iker has never stopped being one of the best keepers in the world, he's always been world-class," he told Marca.
Casillas featured in goal as Real beat Sevilla to lift the UEFA Super Cup on Tuesday and Ronaldo is keen to use that performance as a springboard for a successful campaign.
"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."