Ronaldo suffered a calf injury in Real's 2-1 victory in the first leg in Madrid and club doctors gave him only a 10 per cent chance of being fit for Wednesday's game.
The Brazilian was included in the squad
"I'm well," Ronaldo said on his personal website on Tuesday. "I had a test today and the truth is I feel very well.
"With every hour I'm more optimistic and I now believe I'll be able to play. I'm eager to help the team in a game like this."