The Dutchman has nothing but praise for his former team-mate, and feels the prolific attacker has made it this far because of his hard work
Van Nistelrooy, who was Ronaldo's team-mate at Manchester United from 2003 until 2006, also played alongside the forward at Madrid in 2009-10.
"I never saw him as an arrogant guy. He was always open to advice and not arrogant at all. He was a great guy in the dressing room. He was really vain, but always in a good mood and making jokes," Van Nistelrooy stated to Algemeen Dagblad.
"I talked to him like attackers talk among each other. A lot of his individual actions were not necessary and he lacked efficiency. He still had a lot of room for improvement.
"He worked incredibly hard to become a better player, though. And that's clearly paid off. He really is crazy about the game and always wanted to become the perfect player.
"He's had to work hard to get where he is now. He put in a lot of extra hours after training. And look at where it's got him. He is big, strong, has pace and scores tons of goals. He is a very committed professional."
Cristiano Ronaldo has netted nine goals in as many official appearances this term, and will be looking to add to his tally in Wednesday evening's Champions League encounter against Ajax.