United are preparing for Tuesday's Champions League Round of 16 second leg with the Spanish champions at Old Trafford and Ferguson is well aware what the opposing sides best player is capable of having overseen him during his spell in Manchester.
"(Cristiano) Ronaldo is a supreme athlete. He never misses a game, has fantastic physique, two great feet, pace, he is good in the air," Ferguson said.
"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."
United and Madrid played out a 1-1 draw at the Santiago Bernabeu last month with Ronaldo equalising following Danny Welbeck's opener.