Ronaldo scored his 700th club goal during Manchester United’s 3-1 win against Everton on Sunday.
The 37-year-old has spent two decades operating at the top of the European game, beginning his career with Sporting Lisbon in Portugal before starring in the Premier League during his first spell at Old Trafford.
A move to Real Madrid in 2009 brought further success and a remarkable tally of 450 club goals in just 438 games in the Spanish capital.
Keane was a teammate of Ronaldo’s during his first two seasons at Manchester United, and says he “can’t speak highly enough” of the five-time Ballon d’Or winner.
“He’s been doing this stuff for 20-odd years,” Keane told Sky Sports. “[He’s an] amazing player.
“The longevity is key. That is what makes him great - the amount of goals he’s scored, the different leagues.
“When he first started off, he was a wide player, he wasn’t probably a natural goalscorer, but as the years have gone on he has improved in every aspect of his game.
“His desire - he’s 37 and even his goal record for United is fantastic.
“Everything about him when he first came to United at 17, he had that hunger, that desire. I can’t speak highly enough of him.”
Ronaldo returned to Manchester United last year after a spell in Serie A with Juventus.
The Portugal captain began his career operating out wide but has gradually evolved into more of a centre forward during a career of significant individual and team success.
To go along with his club achievements, Ronaldo is the all-time leading male international goalscorer and helped his nation secure Euro 2016 triumph.
Keane believes that his former teammate’s drive to “learn” and adapt has allowed Ronaldo to continue to succeed.
“I think he has got that football intelligence,” the former Manchester United midfielder explained.
“He is a student of the game, he is always wanting to learn. His record at international level is fantastic, he’s done it with his country, won a major trophy.
“The way he has learned his trade: as he has got older, you could see him change his game. When he was younger he was a dribbler and was about beating players, but as he got older he realised ‘I’m not getting involved in that’.
“He’s just getting on the end of stuff with that football intelligence, a lot of one-touch finishing. Even his mentality to come back to Manchester United. He’s shown real desire to come back and do well for the club. I give him real credit for that.”