Ronaldo has been linked with a return to Manchester United.
"Everyone knows that he is a superstar and a great player," he said.
"The most important thing for the club is that a player of his quality shows the utmost professionalism.
"That is good, especially for the young players who can see a world-class player working with that attitude.
"I am not responsible for the renewal of his contract, but it won't be a problem."