The Portuguese star headed what the Scot could only describe as an "unstoppable" goal to ensure that the Santiago Bernabeu clash ended in a 1-1 draw.
"What a header. Unbelievable. You can't stop that, the leap and the spring to hold himself in the air," BBC Sport quotes Ferguson as saying.
"I said to Patrice Evra at half time, 'why didn't you challenge him?' and then I saw the replays and then thought 'what was I talking about?' We did very well containing [Ronaldo].
"I think it was difficult for him playing against an old team, and it had a little bit of emotion attached to it because he played for us for six years, but he was still a big threat for them. When he gets the ball you are praying more than anything."
Danny Welbeck opened the scoring on the night, giving United the slender advantage of an away goal going into the second leg.