Ronaldo spent six years at United and admits it will be a special couple of games for him.
"As always, it will be a match which he will want to win, a match which he will approach very seriously, because he knows he'll be playing against a team that is one of the candidates to win the competition; and we also know that Manchester [United] are the best English team.
"They're a team that always do well, every year they're at the top, and therefore they're a team we respect a lot - and it will be an extremely difficult match, especially in Manchester.
"And it will be a match... with great emotions, but... I think things will go well. Real Madrid, in my opinion, have a better team, but we have to show that on the pitch, because Manchester are a very united team, and always have been in recent years. So it will be a difficult match."