Portugal forward Ronaldo, sold by United for a world record fee of 94 million euros in 2009, faces his old club for the first time in Wednesday's last 16 first leg tie.
"Ronaldo is a better player than when he left us because of his maturity," Ferguson told a news conference at Real's Bernabeu stadium on Tuesday. "He's at the peak of his career now and the next three years will be the same.
"I never thought he'd ever get the amount of goals he's scored. To score the goals he has scored is phenomenal."
Ronaldo, 28, has been in scintillating form since the turn of the year and warmed up for the United game with a hat-trick in a 4-1 destruction of Sevilla on Saturday, the 17th treble of his La Liga career.
United are riding high with a 12-point lead at the top of the Premier League while Jose Mourinho's Real have all-but given up on retaining their La Liga title after some erratic performances.
"I think it's a good time to be playing Real," said Ferguson. "We've got a fantastic lead in the league and we have everyone fit.
"When you come to this part of the season you usually have one or two or three players injured. We don't have that, which is a bonus.
"It's a shame we're meeting this early in the competition. I wish it had been [in the final] at Wembley."
United have won the European Cup three times. Their last triumph came against Chelsea in 2008 before they lost to Real's arch rivals Barcelona in the final in 2009 and 2011.
Ferguson said he was convinced his side could go one better this season.
"There's a great spirit about them," he added. "The reality is this team doesn't know when it's beaten, which is a great quality.
"The acid test is tomorrow and we have to get through it if we are to win this trophy."
Ferguson said Japan midfielder Shinji Kagawa would feature at some point on Wednesday.
"We didn't play Shinji [against Everton] on Sunday because he had a long trip back from Japan," the manager explained.
"I wanted him fresh in case I decided to use him tomorrow so I rested him. He will certainly be involved at some stage tomorrow."
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