Ronaldo goes into the game on the back of a Champions League brace in midweek.
"How do you stop Ronaldo? With a rope. I don't really know, we will see during the game," Ruben said.
"Real Madrid are in a great moment of form and they are getting some good results. They also qualified from their Champions League group.
"However, despite Real Madrid's strengths, we will play the game with everything we have."