Ronaldo became a household name in football from the late 1990s to mid-2000s despite a series of severe knee injuries which curtailed his promising career.
He won two World Cups with Brazil and personally won three FIFA best player of the year awards.
Ronaldo was nicknamed ‘El Fenomeno’ during the apotheosis of his career due to his goalscoring exploits.
Messi and Ronaldo have raised the bar of individual brilliance and are widely considered as two of the greats, but Carlos, who played with Ronaldo between 1994 to 2007 for club and country, believes his compatriot is the greatest of all-time.
"Ronaldo stood out in training. He was the best. There will never be another Ronaldo 'Fenomeno'," the former Inter Milan defender told Goal. "Not Neymar, Cristiano or Messi – Ronaldo is unique.”
"I think in our generation it was harder to score goals. It was more physical back then and attackers were less protected. But Ronaldo could do everything."
Ronaldo shook off the niggling knee injuries and made his mark during the 2002 World Cup in Korea/Japan before signing for Real Madrid in the summer.
Carlos recalls when Ronaldo proved his doubters wrong by scoring two goals on his La Liga debut against Alaves.
"I knew how unbelievable he was when he arrived but a lot of people at Madrid didn't," the 47-year-old added.
"He surprised a lot of people. I remember 80,000 people were very happy and pleasantly surprised by him."
