The striker, 36, signed from Juventus on a three-year-deal following a 12-year hiatus from wowing the crowd at Old Trafford.
Having excelled for nine years at Real Madrid by scoring 450 goals in 438 games and producing a similar ratio at Juventus, Ronaldo felt the time was right to move back to the club where he made his name.
He could make his second debut against Newcastle on Saturday, with fans across the world expectant he will score against the Magpies.
But the games where he will be under the spotlight most will come against United's rivals.
Cristiano Ronaldo has moved back to Manchester United for a second time (Image: GETTY)
It will be interesting to see who can keep Ronaldo out, given his record against clubs in the past.
Newcastle especially will be anxious that Ronaldo will put on a second-coming show against them should he make his debut on Saturday.
With six goals and five assists in just eleven previous games against Newcastle, Ronaldo will look to carry on his one-goal-per-game contribution for Ole Gunnar Solskjaer.
Elsewhere, Arsenal are also big victims of his talent - with six goals and two assists conceded in 15 matches against the Portuguese, many when he was but a fledging facing the great invincibles side.
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However, fans will look to see how he does against Liverpool and Manchester City - and they might be surprised at the record on show.
Ronaldo has notoriously struggled against Liverpool, with just three goals in 12 matches against the Reds.
He came across a proficient Liverpool side, albeit a side that very rarely threatened to win the Premier League, and could struggle further against the likes of Virgil van Dijk.