It had been 15-and-a-half years since the most decorated sides from England and Italy last met in a competitive fixture, but anyone watching Tuesday's Old Trafford encounter will be under no illusion who is now in the better health.
Juventus toyed with United at times as Ronaldo enjoyed a winning return to his old stomping ground, with his cross leading to Paulo Dybala's goal on an evening that sometimes threatened to be far worse worse than the eventual 1-0 scoreline.
For Manchester City, things are looking much better. They began their Champions League campaign with a home defeat against Lyon, but have responded impressively. Their 3-0 win over Shakhtar sees them move top of Group F, a point above Lyon and four ahead of Hoffenheim.
A win for Juventus in the return fixture in Turin will be enough for them to win Group H with two games to spare. For United, qualification is still not secure, although they were helped by Valencia's draw at Young Boys earlier this evening. What is clear is that improvements will be needed if Mourinho's side are to advance into the latter stages of the Champions League.
The focus was on Ronaldo tonight but it was Dybala, and Juventus' sturdy defence, earning the plaudits. The Portuguese did, however, need the help of security when a fan appeared to rush at him immediately after the final whistle. The reception from the fans, though, was positive.
That's it! A huge win for Juventus, who go five points clear of United at the top of Group H. Disappointing from United, who improved after the break but never truly looked like breaking down their stubborn opposition.
The visitors break quickly in search of a conclusive second. Ronaldo lays the ball off to substitute Douglas Costa, whose pull back is deflected out for a corner. United, eventually, deal with the set piece.
Mata finds space on the right and plays the ball into the box. Lukaku tries a first time shot but it's blocked and the Belgian's frustrating night continues. Time's running out for United.
The result in Ukraine is inevitable; it's just a case of how many City can score. Sterling weaves past three defenders in search of a fourth, but he's crowded out.
United might be relieved to see the back of Dybala, who's withdrawn for Federico Bernardeschi with just over ten minutes remaining. Max Allegri might be feeling a bit tense as the game reaches its conclusion. His side, unlike in the first half, have not always been convincing.
Ronaldo is lurking in the box, waiting for a moment to strike. He nearly gets it as Bentancur lifts a ball into the box. De Gea is out quickly to claim the ball.
United have been better in the second half, without doubt, but frustration is beginning to grow at Old Trafford. Juventus are stingy defensively and United's lack of creativity in the final third is in evidence.
He's only been on the pitch for a couple of minutes, but Bernardo Silva adds to City's tally with a low drive into the bottom corner from 20 yards out. What an introduction! 3-0.