"It is not something we should fear. If we worry about the danger Cristiano can give us we will forget some of the things we can do ourselves," Ferguson said.
Ronaldo, who returns for the first time since his record transfer move in 2009, scored Real's equaliser in the 1-1 draw at the Bernabeu.
Ferguson feels Ronaldo can be the biggest threat to United's chances of qualifying to the quarter-finals.
"What do you expect when you play against a team with Cristiano in it?" said Ferguson. "You expect problems during the match. We have to try to curtail that as much as we can and as best we can," he said.
United right-back Rafael was given the task to stop Ronaldo flying down the left flank in the first leg and the young Brazilian defender was up to the mark.
"The experience young Rafael had in the first half in Madrid hopefully helps him because he will be given the same task again," Fergie said.
Real coach Jose Mourinho described the second leg as "the world will stop to watch" and Ferguson feels "it doesn't get any bigger than this one."
Mourinho's men are heading into this tie with back-to-back wins over arch rivals Barcelona while United have been consistent throughout the season.
"Both teams are in form. Real have had two fantastic results against Barcelona. I saw the first one and they were absolutely superb. They have found form at the right time and our form has been consistent for quite a while," the 70-year old Scot said.