Ronaldo will play against the Red Devils for the first time since moving to Real Madrid when the two sides clash later today.
Ferguson told reporters: "Ronaldo is the one to worry about. I knew he'd improve, I knew he was one of the best players in the world, and probably he and Lionel Messi are the two best players in the world.
"But to score the goals he has is just phenomenal. I would think he's at the peak of his career now, and the next three years will also be that way for him."
The 28-year-old is in great form heading into the game and has scored 10 goals in his last five games, including two hat-tricks.