Ronaldo, 28, spent six successful years at Old Trafford, during which he scored 117 in 290 appearances, including 42 goals in 2007-08 as United won the Premier League and Champions League.
On Wednesday, four years after leaving Manchester to join Real Madrid for a world record £80m, the Portugal international will line up against his former club, looking to add to his tally of 182 goals in 179 games for the Spanish club.
"Cristiano is a better player than when he left us because of his maturity," said Ferguson. "He is at the peak of his career now.
"He was still a young man when he left and you can see how he has flourished in Madrid. His fitness, speed, goals - and he's never injured. That is unique in the modern game.
"I never thought he'd get the amount of goals he's scored. I knew he'd improve. I knew he was one of the best players in the world. But to score the goals he has is phenomenal."
However, United have their own attacking threats.
The major difference for United in the title race this term has been the arrival of Robin van Persie, who on Sunday took his goals tally for the season to 23 against Everton.
"Robin is now approaching that level," said Ferguson of comparisons with Barcelona's Lionel Messi and Ronaldo. "He has been a breath of fresh air."
Van Persie's strike-partner Wayne Rooney was depicted as a "hooligan" in the Spanish press on Tuesday, with sports daily Marca also describing the England player as "a freckled demon" who is "built like a barrel packed with gunpowder" and on his way to "blow up the Bernabeu".
"He is a mature player now," said Ferguson of Rooney. "He is 27. When players mature little bits of their games change.
"His goalscoring has been very consistent. He used to score in snatches. Now there is a far more consistent nature to his goals.
"He also plays different positions with great enthusiasm. Not every player in the world has these qualities."
Asked about today's unflattering reports in the Spanish press, Ferguson said: "Rooney can't read Spanish so we will be all right."
The United manager admits he's been looking forward to the visit to Madrid since the two sides were paired and said: "I think as soon as the draw was made, there was that anticipation about the tie.
"It's a fantastic challenge for myself and it should be a fantastic match between two teams and clubs who embrace the best parts of football, with great reputations.
"The acid test is this game, I have no doubts about that.
"It is unfortunate that we are meeting as early as this. I wish it had been at Wembley (in the final in May)."
While United have opened up a 12-point gap over rivals Manchester City at the top of the Premier League, Real have endured a turbulent domestic season and languish 16 points behind foes Barcelona.
Claiming the pressure would be on for Jose Mourinho to deliver in this competition, the Scot added: "I think Jose has set his targets firmly on the European Cup, no doubt about that.
"It can be hard for us but there's a bit of pressure for them, too. At this moment in time, we're involved in three (competitions)."
You can only watch the best UEFA Champions League coverage here on Sky Sports. Watch Real Madrid v Manchester United exclusively live on Sky Sports 2HD and Sky Go from 6:00pm on Wednesday 13th February. Click here for more details and watch goals from United's route to the last 16.