The Portugal international believes that the attacking midfielder is one of the best players in the world, and therefore belongs at the Santiago Bernabeu with the Liga champions
The Brazilian midfielder is continually being linked with a move back to AC Milan, but the Rossoneri's dire financial situation has so far held up a deal, and his team-mate does not think the deal will materialise before the transfer window closes.
"Whether Kaka will leave the club or not is a decision he has to take himself, but I think he will stay here," Ronaldo told reporters.
"For me his is one of the best players in the world, and all the best players have to be at Real Madrid."
The Portugal international then looked ahead to Madrid's Liga opener against Valencia, and stressed they want to start the season on top form.
"We want to keep improving because it will be a long season. What we all want is to start the season in the best way. We are working well together and we want to start the league on form."
Madrid host Valencia at the Santiago Bernabeu on August 19.