The first leg between the two clubs ended in a stalemate and Ronaldo admits the second leg at Old Trafford will not be an easy game.
“We have to score and we have to win,” Ronaldo told The Sun.
“It won’t be an easy game, but I am optimistic because I know we can do it.
“In the first leg we had lots of good chances and we should have taken advantage of them.
“But we will have more opportunities and the fans will be on our side.
“It’s hard to win the Champions League, but the feeling is magnificent and I want to relive that.”