Manchester United and Real Madrid face each other at the Santiago Bernabeu on Wednesday night, with the former Old Trafford star playing against his old team for the first time
Sir Alex's side face the Liga champions at the Santiago Bernabeu, with the match representing the first time former United man Ronaldo has played against the club with whom he made his name.
Although behind Barcelona's Lionel Messi in the Liga scoring charts, Ronaldo has been in superb form this season with 24 goals, however, Sir Alex believes Rafael will be fearless in the face of the Portugal star.
Speaking to MUTV, Ferguson said: "The responsibility will be with Rafael. 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."
Since selling Ronaldo for £80 million, Sir Alex believes the Portugal international has developed even further but adds that if any team know how to stop him, it is United.
"He's flourished since he's come to Madrid," the 71-year-old said. "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."