Ronaldo picked up an injury while scoring during Sunday's 2-2 draw with bitter rivals Barcelona, eventually being forced off at half time of El Clasico as Barca maintained their hopes of going through an entire season unbeaten despite playing 45 minutes with 10 men.
Zidane admitted that the exact severity of Ronaldo's ankle problem is as yet unknown, but he is confident it is only a "small" issue and insisted that the five-time Ballon d'Or winner will be fit for the final on May 26.
"It doesn't feel good right now, but I think it's going to be small [problem], it's just a knock," he told reporters.
"Don't worry about the final, he will be there. I cannot tell you how long [he will be out for], but he said that he thinks it isn't much. We will see tomorrow.
"We will see what we have to do, and the damage that has been done. It was a little swollen, but as I say, it wasn't much."
Ronaldo could now be rested for Madrid's final three league games of the season, which come against Sevilla, Celta Vigo and Villarreal.