The Portuguese was consistently linked with a return to Old Trafford last year before putting pen to paper on a new deal which runs until June 2018, by which point he would have spent nine years at the Santiago Bernabeu.
Ronaldo, who will be 33 years old by that point, claimed in November that he would like to stay with the current European champions for longer than his existing deal if they will have him and his agent, Jorge Mendes, is convinced that the Portugal international will be a Real Madrid player until the day when he hangs up his boots.
"I got to know him when he was 15 years old. From then until now we've spoken together on most days," he told Globo Esporte. "When I'm in Madrid we always meet up. Our relationship is more like a friendship.
"I believe that Cristiano will finish his career at Real Madrid. He's happy at the club and is still keen to achieve more success in Madrid.
"Will the club retire his shirt number when he finishes at Madrid? With every respect to the likes of Raul and Di Stefano, no player can compare to Cristiano Ronaldo. He's the best player in history and, I'd suggest, the best sportsperson of all time - we'll never see the likes of Cristiano again. It will be completely impossible."
When asked if overtaking Barcelona forward Lionel Messi's tally of four Ballons d'Or, Mendes responded: "If he stays fit, I'm sure he'll remain the best for some time."
Neymar mused over Christmas that he might never reach the level of Messi and Ronaldo but Mendes thinks that the Brazilian is destined to be among football's greatest soon.
"He's not quite Cristiano but he is fantastically talented," the agent continued. "I'm confident that he'll be amongst the best in the world in a few years. He has been unlucky that he has not ended up playing alongside Cristiano."