In his six years at the Theatre of Dreams, the Portuguese won the Premier League three times and the Champions League once en route to becoming one of the best players in the world.
He won the Ballon d'Or in 2008, which sparked a long-lasting battle for supremacy with Barcelona's Lionel Messi.
Ronaldo then joined Real Madrid in 2009 and he departed United as one of their greatest ever players - maybe even the best.
Among other things, it was Ronaldo's work ethic and determination to be number one that set him apart from his peers.
Extra-long training sessions, hours spent in the gym and a strict diet turned him into a machine and even at 34 years old at Juventus he remains in peak condition.
If one story sums up Ronaldo's obsession with healthy eating and wanting to improve, it's the one Patrice Evra told last summer of going over to his house for lunch one time.
Ronaldo and Evra had just completed a training session - presumably for United - and yet Ronaldo was in no mood to relax as he rushed through lunch and made Evra play two touch with him.
Take it away, Patrice.
"I would give advice to anybody, when Cristiano invites you for lunch at his house, just say no," the Frenchman said in 2018, per Marca.
"He said: 'Patrice, come over after training.' I went [and] you know I was really tired. At the table there was only salad and plain white chicken so I was like 'okay...' and water, not any juice.
"We started eating and I was thinking some big meat would be coming after that but there was nothing.
"He had just finished and he stood up and he started playing with a ball, doing some skills and he said: 'Let's do some two touch.'
"I was like: 'Can I just finish eating?' [Ronaldo replied:] 'No, let's play two touch.'
"We start playing two touch. After that he said let's go to the pool to swim, I was like 'okay...' after going in the jacuzzi, the sauna, I'm done.
"I said: 'Cristiano, why have we come here, have we come here because we have a game tomorrow, or just for lunch?'
"So that's why I would recommend to anyone when Cristiano invites you to his house, don't go. Just say no because this guy, he's a machine, he doesn't want to stop training."
Classic Cristiano Ronaldo.News Now - Sport News