The Portugal star is facing the Red Devils for the first time since his £80m move, and his former Old Trafford boss thinks the 28-year-old is more talented than the French legend
The Portugal forward became the world's most expensive player when he made his €94 million move from Old Trafford to the Bernabeu in 2009 after winning the Ballon d'Or in a glittering career with the Red Devils.
And while the 71-year-old manager will be hoping his former prodigy can be kept quiet enough for the Premier League leaders to progress in Europe, he thinks the 28-year-old's skills surpass even those of the famous 'Zizou.'
"Florentino Perez had all the Galacticos at Real Madrid when he was president first time around," Sir Alex told reporters. "He had [Luis] Figo, Zidane, Roberto Carlos, Raul in that spell. But I don't think any of those players were as good as Ronaldo.
"Zidane was a fantastic player, Figo was a fantastic player. But not as good as Ronaldo. So I think Mourinho's team are still living up to the reputation that Real Madrid can produce the top players.
"Ronaldo has beaten all the records there. He's beaten [Alfred] Di Stefano, [Ferenc] Puskas, Hugo Sanchez. So obviously he is indelibly printed in their history now.
"You have to say he is pivotal to all they do - 179 goals in 178 matches tells its own story. I think I might have to think about blackmail. I might have to tell a true story about him!
"He's a fantastic player. I knew he would improve tremendously, he was still young when he left us."
Sir Alex says Ronaldo's dedication to training allied to his fantastic talent sets him apart from other players, and even suggested the world record fee they paid for him was a bargain for Madrid.
"He would practise half-an-hour after every training session," he continued. "It was a simple route to becoming a complete footballer. The more you practise the better you become, and it became a habit for him.
"I told them upstairs [the Glazers]. We should have asked for €175m; we probably would have got it."
But despite the Scot's clear admiration for the Madrid star, he does not believe he will ever be able to afford to sign him again.
"You are talking hundreds of millions; I don't see it," he added.