Ronaldo spent six seasons at the Red Devils and scored against his former club in the reverse fixture at the Santiago Bernabeu a fortnight ago.
United's away goal in the 1-1 draw gives them a slender advantage heading into the last-16 decider, but Mourinho is confident that his star man, whom he believes to be better than Diego Maradona and Pele, can inspire Los Blancos to victory.
He told ITV Sport: "How do United stop Cristiano Ronaldo? That's not my problem. It's never a problem – I never want to stop Ronaldo. It's their problem.
"He is the best. The best in the world, yes. Probably the best ever. I saw Maradona a couple of times. I never saw Pele. But Cristiano is amazing. This man is the best."
Ronaldo, who has scored 34 goals in 39 appearances this season, was pipped to the 2013 Ballon d'Or by Barcelona's Lionel Messi, who has netted 46 times, having played three less matches.