Ronaldo is the current Ballon d'Or holder and widely expected to retain the prize on the back of a prolific year in which he fired Real Madrid to Champions League glory.
However, while Capello has the utmost respect for the Portugal international's explosive power, the Italian coach does not believe that the 29-year-old is as skilful as the Barcelona No.10.
"Cristiano Ronaldo and Messi are two extraordinary players but they are different," the former Madrid boss told AS.
"Cristiano is a powerful player, with very good technique, but he does not possess the pure technical skill of Messi. Leo Messi is a very difficult player to play against."
Indeed, Capello revealed that he first came to realise just how much natural talent with which the Argentine was blessed during former club Juventus's Gamper Trophy clash with Barca in 2005.
Messi terrorised a Bianconeri defence led by Fabio Cannavaro, prompting Capello to take the unusual step of making an approach to sign the then-teenager while the game was still going on.
"It's completely true that I asked [then-Blaugrana boss Frank] Rijkaard if he would loan me Messi," the 68-year-old confirmed. When the play had stopped for a minute, I went over to Rijkaard and said: 'You can't play him because you've got three foreigners already. Why don't you let me have him for a year?'
"Rijkaard replied: 'We're going to clear up the situation in three or four months and for that reason I can't do it.'
"That's why Messi's [Barca] debut was last in coming. I knew the value of my defender and when I saw Messi play I knew he was special. I could see that he could do things that didn't occur to other players."
Messi, a four-time Ballon d'Or winner, is poised to become the Spanish league's all-time top goalscorer.