The 36-year-old has terrorised teams in England, Spain and Italy at club level and also on the international stage.
He has returned to United and is in contention to start against Newcastle this weekend in their 3pm kick-off at Old Trafford.
Ronaldo's goal record speaks for itself. He has 674 in 897 games at club level and another 111 in 180 matches for Portugal.
So what is it like to actually for the five-time Ballon d'Or winner? Former Aston Villa striker Gabriel Agbonlahor came up against Ronaldo several times during his first spell at United between 2003 and 2009.
"He can outpace you, beat you in the air, score with his left foot, right foot - unplayable is the word I would use," Agbonlahor told talkSPORT.
"There's a massive aura about Ronaldo. His physical presence and build of muscle add to that aura of 'I'm Ronaldo, I'm going to go out there and play.'
"With players like Ronaldo, if he did score two or three goals past you or he did 'meg you or get past you with a bit of skill then you could still have your head raised high because it's Cristiano Ronaldo.
"It was like an honour, as a defender for him to 'meg you.
"I don't think most of them, after playing Ronaldo, could take their boots off for 20 minutes after because they were still shattered from the game."
When asked to describe to superstar in three words, he said: "Electric, goal scorer, professional."
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Ronaldo's return has created a huge buzz around the Premier League, much like Lionel Messi's move to Paris Saint-Germain.
Agbonlahor added: "Ronaldo's signing will be massive for the Premier League like what Lionel Messi's signing has done for the French league.
"Ronaldo has fans across the world who are obsessed with him and will be tuning in more to the Premier League more now that he plays there.
"I think everyone who is a neutral [supporter] will be looking forward to seeing him play."
While plenty of supporters can't wait to see Ronaldo in action, there are even some players who are equally as excited to see him back in the Premier League.
"I think everyone is a little bit giddy," Everton ace Andros Townsend told talkSPORT.
"My era growing up 15, 16, 17 was Ronaldo's prime at Manchester United, scoring all those goals, inventing a new way to kick a ball.
"He was our idol growing up. It's amazing for me to hopefully share the pitch with him."
Many of us were eagerly following the updates on Ronaldo's sensational move back to United, and the Everton players were just as gripped.
"I think we were travelling for an away game and we heard the news of Ronaldo to Man City," Townsend explained.
"So obviously we were discussing everything that goes with that and then we turned on our phones, and bang, the news has broke that he's signed for Man United.
"It was literally all we were speaking about."
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