The Portuguese forward left the Red Devils for a world record £80 million transfer fee in 2008 and has since established himself as a dominant force in La Liga - netting 136 goals in 123 league appearances.
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"Ronaldo is the one to worry about," the Manchester United manager told the Daily Mirror.
"He’s a mature player now, he’s 28. When players mature, little bits of their game change in terms of consistency.
"His goalscoring has been very consistent. He used to score in batches, a few goals together, but he’s now more consistent.
"He can play in more positions, he has energy, enthusiasm and desire. He’s a terrific player."
Sir Alex believes Ronaldo’s performance and overall work rate has improved since his time at Old Trafford, ranking the former Sporting Lisbon player and Lionel Messi as the two greatest players in the world.
"He was still a young man when he left me," Sir Alex explained.
"But I think you can see how he’s flourished in Madrid and his record is - well, I never thought he’d get that total, to be honest with you.
"I knew he’d improve, I knew he was one of the best players in the world, and probably he and Lionel Messi are the two best players in the world. But to score the goals he has is just phenomenal.
"I would think he’s at the peak of his career now, and the next three years will also be that way for him."
Manchester United make the trip to Real Madrid on Wednesday evening with Sir Alex anticipating a lively game, but he has lamented two club of huge stature meeting so early in the competition.
"It won’t be 0-0, it definitely won’t be," he said. "Of course it can live up to expectations.
"It’s unfortunate to be meeting so early, I would have preferred it in the final at Wembley, obviously, but this is so important to both clubs. I cannot think of failing."