Who knows how Rangnick's six-month stint as interim manager at Old Trafford will ultimately turn out, but two goals from Ronaldo -- the 800th and 801st of his career -- in a 3-2 win against Arsenal proved that he simply cannot be overlooked by Ole Gunnar Solskjaer's temporary successor.
Rangnick, the former Schalke, Hoffenheim and RB Leipzig coach, will officially replace caretaker-manager Michael Carrick, who announced his departure from the club after the game, when he takes his first training session on Friday. The German watched from the directors' box as his new team emerged triumphant in an eventful clash against the Gunners and he will have left with his notebook full of issues to address.
He needs to deal with a goalkeeper in David De Gea whose fragility in a packed penalty area once again contributed to a goal being conceded when, in farcical circumstances, Arsenal took the lead after Emile Smith Rowe scored while the Spaniard was lying on the floor following a "foul" by United teammate Fred. VAR had to confirm the visitors' goal due to the confusion surrounding the incident.
And aside from De Gea, who went on to make a series of outstanding saves, Rangnick will be concerned about the quality in both full-back positions, Fred and Scott McTominay's limitations in midfield and the inconsistency of Marcus Rashford up front.
But while Ronaldo, who is 37 in February, cannot expect to play every game between now and the end of the season, his performance against Arsenal will have banished any doubts Rangnick may have held over his value to the team.
As the man credited as being the so-called "Father of Gegenpress," the high-energy pressing game which requires forwards to be the first line of defence, Rangnick is unlikely to abandon his philosophy simply to accommodate Ronaldo in his team.
Rangnick may bend slightly and make allowances for Ronaldo because of his peerless ability to score crucial goals, but it is the player who will have to measure up and prove himself in the end.
Ronaldo's sharp exit down the tunnel at Stamford Bridge following Sunday's 1-1 draw against Chelsea, when he was dropped to the substitutes' bench by Carrick, highlighted how the former Real Madrid and Juventus forward will never accept a bit-part role.
But if anyone was expecting Ronaldo to sulk in response to being dropped, he did the opposite in a man of the match performance against Mikel Arteta's team.
His goals are almost expected now, but the accusation levelled against Ronaldo is that he doesn't work for the team -- any team -- and that everything has to go through him and be devised to suit his strengths and weaknesses.
Rangnick saw a different Ronaldo than the stereotype, though. The Portugal captain chased down defenders, forced them into mistakes and gave a perfect display of "gegenpressing."
His work rate was exceptional. Two moments late in the game highlighted the effort and determination he displayed.
The first saw Ronaldo close down defender Gabriel and bundle him to the ground on 85 minutes. It was a foul and he conceded a free kick, but that Ronaldo was prepared to do that in the closing stages showed he is ready to what Rangnick wants.
Then two minutes later, Ronaldo headed an Arsenal corner clear at the near post to launch a counter-attack and he proceeded to sprint to the other end of the pitch in a desperate effort to score his hat-trick goal after earlier netting from a Rashford pass and a 69th minute penalty.
Ronaldo was exhausted by the end of his run and was immediately substituted, but if he had to prove a point, he did that in typical fashion.
With a game against Crystal Palace at Old Trafford on Sunday, recovery will be the key for Ronaldo over the next 48 hours. But Rangnick should do all he can to ensure that the Portugal captain is fit and fresh to start his first game in charge.
And he can thank Carrick, leaving the club 15 years after arriving as a player from Tottenham Hotspur, for the winning platform he can now build upon.
Under Carrick, United have won twice and drawn once to halt the run of defeats under Solskjaer. Bruno Fernandes' equaliser in the first-half, prior to Ronaldo's second-half double, proved enough to win the game, despite Martin Odegaard making it 2-2 on 55 minutes.
Confidence is returning, but United remain a team in need of structure and organisation. Delivering that is now Rangnick's job, but at least he knows he can rely on Ronaldo.