Real's last visit to Old Trafford saw former Real striker Ronaldo score a hat-trick.
"The older one, the fatter one, peaked as a centre forward at that time," said Ferguson.
"(Cristiano) Ronaldo is a supreme athlete. He never misses a game, has fantastic physique, two great feet, pace, he is good in the air.
"My biggest concern is if he turns up because you expect problems.
"We have to try and curtail that as best as we can. It won't be easy because he does it every week.
"But it isn't something we should fear. If we go in worried about the damage Cristiano can do we forget what we can do ourselves."
Can Ronaldo match the feats of the 'fat one' tonight? How will United fans react if he scores the winner? Give us your shout below...