The Manchester United manager has also joked to reporters that he might resort to underhand tactics in a bid to shackle his former star during Wednesday's UEFA Champions League last-16 tie in Madrid.
“Ronaldo’s form, I have to say, is pivotal to everything Real Madrid do," Sir Alex said. "His goal tally - 179 goals in 178 games - tells its own story. I think I will have to bring a bit of blackmail to it and tell the true story of him!
"He's a fantastic player and I knew he would do well and improve tremendously in Madrid. He was still youngish when he left us and it was obvious he was going to be a top player in the world, no doubt about that.
“When he first came, he was a boy who wanted to display all his tricks and technical abilities but, bit by bit, he overcame all the little flaws in his game because he’s an absolutely rounded player.
“His last game of his last season for us... well in almost every one, he created five chances, sometimes more, by himself. He was unbelievable.”
Earlier this season, Ronaldo sent ripples throughout football by telling reporters "I am unhappy and the club knows why", prompting rumours of an imminent departure from the Spanish capital and a possible return to United, where he made just under 300 appearances during six trophy-laden years.