The Portuguese is a major doubt for the clash at the Bernabeu following a period on the sidelines with a knee complaint, and head coach Carlo Ancelotti is not expected to make a final decision until the last moment.
Alonso, however, is hopeful that the Ballon d'Or holder is fit to take part.
"If he is 100%, he will play but that's a decision for the coach, the player himself and the medical staff. We need him, he's an important player for us," he told reporters at a press conference.
"Cristiano gives us an explosive edge, the potential to create chances and score goals. He's a fundamental player for us and when he's in the side, we make the most of the opportunities he creates for us. The other day in the Copa del Rey final, you could see that the whole team delivers."
Ronaldo has scored 14 times in the Champions League this season.