The Portuguese superstar's leaping first-half header had cancelled out a similar effort from Danny Welbeck that gave United the upper hand.
But Ferguson was quick to hail Ronaldo's equaliser which has set up a mouth-watering second leg at Old Trafford on March 5.
"What a header. Unbelievable. You can't stop that," said the Scot of the Portuguese's goal.
"The spring, the way he holds himself in the air, the power in the header. I said to Patrick Evra at half-time, 'you didn't challenge for that'. And when I saw the video I thought, 'what was I talking about?'.
"I think it's difficult for Cristiano playing against us, his old team. There's a little bit of emotion attached to it for him because he had so many great years for us.
"But when he gets the ball you're praying, more than anything. He could undo us in the second leg."
Assessing a busy 90 minutes, Ferguson added: "There were a lot of incidents in the game. We had lots of chances in the match and played some good attacking football at times ,but we did not get the possession we should have had.
"Real won possession without question, but it takes us back to Old Trafford with a great chance.
"It will be very open. I think we'll score at Old Trafford. We have a big job on our hands but it is definitely within our grasp."
But Real Madrid coach Jose Mourinho is convinced the tie is still in finely balanced at "50-50". Asked whether the draw handed the advantage to United, he replied: "No. We can score more than one goal there."
He added: "They had two big chances in the second half, but chances out of context because we were chasing the game and they were just defending.
"This is the Champions League and there are two matches, home and away. They did their match and they did it well."
United striker Robin van Persie, who might have had a hat-trick on another night after spurning some inviting openings, was cautious in his assessment of the result.
A draw on the road, complete with an an away goal, represents a fine return for the Red Devils. But the Dutchman does not believe the fact that the sides head to Old Trafford for the deciding match will be a critical factor.
He said: "It was a good result away from home. People who have watched the game have seen chances for both sides. It was a great game to watch."
Reflecting on his own chances, Van Persie admitted he was surprised to be leaving Madrid without a goal to his name.
"I had three opportunitiesin the second half, especially one where I hit the bar. It was a big chance and I should have put it in. With a bit of luck it could have gone in.
"Then in the dying seconds the keeper (Diego Lopez) made a great save. I was counting on that one going in."