The Portugal forward picked up an injury to his right ankle in Real's 3-0 victory over Olympique Marseille at the end of September and aggravated it while on international duty soon after.
"He's looked in good shape in training over the last few days. We'll see how much he will be able to play tomorrow," Real coach Manuel Pellegrini told a news conference.
The Chilean refused to be drawn on whether he would start, or how many minutes Ronaldo might play, but welcomed the return of the world's most expensive player.
"I've always said it's impossible not to miss a player of his quality. He's regarded as the best in the world," Pellegrini added.
"But you cannot rely on one player and the team have responded well in his absence. We are top of La Liga and (joint top) of our Champions League group."
Ronaldo is expected to get a run-out at some point against Zurich in preparation for the trip to face Barcelona in La Liga on Sunday.
Real are level on seven points with AC Milan in Group C, and can qualify for the knockout phase with a victory if Marseille lose against Milan. Zurich are bottom with three points.
