Ferguson was considering a number of options, including using Phil Jones to man-mark him having done so well in recent games against the likes of Marouane Fellaini and Gareth Bale.
However, Ferguson says Rafael will be the man who is given the job of keeping his former team-mate quiet.
"The responsibility will be with Rafael," Ferguson told MUTV.
"Rafael's maturity this season has been fantastic. Obviously Robin van Persie has been sensational but, outside that, Rafa and Michael Carrick are having their best seasons ever.
"He's Brazilian so he doesn't fear a thing. I trust him and have faith in him tonight."
Ferguson admits that Ronaldo does need special attention, saying: "He's flourished since he's come to Madrid.
"You'd have to be stupid not to respect his goalscoring record. But if one team knows about Ronaldo it's us.
"We had him for six years so we know everything he can do on a football pitch. We probably have an advantage in that respect, when it comes to facing Ronaldo."