The Portugal international has been struggling with a thigh injury of late, but Mourinho is confident that his compatriot will start at the Santiago Bernabeu in what he claims will be "the most important match in 10 years" for the Spanish club.
"I respect Real Madrid's history a lot," he told reporters on Monday. "It is difficult to say what mathematical chance we have. I've already said everything to my players. The most important thing was to talk about the mistakes we made in Dortmund.
"The result was fair in Dortmund and Borussia deserved to win. Cristiano is OK, but [Alvaro] Arbeloa and Marcelo are injured. It's only football and everything is possible. We have to enjoy ourselves on the pitch, although we are in a difficult situation.
"We have to play it minute by minute and goal by goal until the last breath of the match. I feel this is the most important match in 10 years for Real Madrid, but I also felt that in Dortmund and we played like [it was] a friendly. I have considered Borussia a candidate to win the competition since the beginning of the season."
Madrid lost the first leg 4-1 in Germany last week.