The Portugal international has been in scintillating form this season, and Ferguson admits that there are not many ways to keep Ronaldo quiet.
"He is a better player now than he was when he was with us because he has matured. He is 28 and at the peak of his career," Ferguson told ESPN.
"From 28 to 32, with the fitness Ronaldo has got - he never misses a game and is never injured - he will be at his absolute best.
"I've got a plan to stop him: it's called a machete. Plan B is a machine gun!"
United host the La Liga champions at Old Trafford on Tuesday.