If they don’t, they’ll get a thrashing’ to match the 2-0 defeat of last November.
The one that could have been 6-0 if Pep Guardiola’s team had been bothered to go after that many goals.
The sight of City toying with United on that grim afternoon at Old Trafford was one of the biggest results that cost Ole Gunnar Solskjaer his job as manager of the Red Devils.
It’s one thing to lose to City, to lose while your opponents are going through the motions, barely bothering to attack, is something else.
United’s players could not even get the ball off City that afternoon, they were left chasing shadows.
It was a chastening experience for everyone connected with United. The game showed the chasm in class between the teams.
On Sunday at the Etihad, United’s XI simply have to put a shift in, they have to be right on top of their men and the ball.
They have to graft, tackle, cover for each other, chase down lost balls and do something to stop this superb City side from playing the way they want to play.
Above all, it is time for the main man to step up. Games like Sunday’s were made for Cristiano Ronaldo.
In the context of what is going on in the world, it seems crass to be talking about this being the biggest football game staged in the world today.
But that’s what it is, and thus it is an ideal occasion for Ronaldo to score a goal or two and make up for some below-par efforts of late.
His team-mates have to do better too, much better.
Or else Kevin de Bruyne, Riyad Mahrez, Phil Foden and whoever else from his star-studded cast Pep decides to play will run riot.
If United were to get even a draw, it would help Liverpool in their title bid.
And United don’t like helping anyone from Anfield in anything.
But they don’t like being made fools of by the ‘noisy neighbours’ either.
So United should just get on with playing their football and not worry about who ends up better off in the title race.
And anyway, after the disastrous home draw with Watford last weekend, United have to worry about themselves.
They need every Premier League point they can amass if they are to hold off Arsenal, West Ham and Spurs in the race for fourth place in May.
Mind you, the fact that United might finish fourth is a condemnation of the standard of this season’s top flight in England.
United have not played well at all, and yet they are bang in there with a chance of qualifying for next season’s Champions League. And soon they could be even higher than that.
For what happens if the new owners of Chelsea are not willing to put in the £1.6billion that Roman Abramovich has invested at Stamford Bridge over the last decade, and is now willing to write off just to get a sale of the club?
Will Chelsea slip off the pace now being set by City and Liverpool at the summit, and drift back into the pack?
What happens when Chelsea can no longer spend £97milllion on a centre-forward like Romelu Lukaku, who doesn’t seem to be the manager’s type of main attacker?
These are just some of the questions that have emerged.
Right now, you just know it is a long way back for Manchester United to get to the standard of this season’s ‘Big Two’.
Whoever comes in as manager in the summer is going to need a big cheque book, and to be able to clear out of some of the deadwood at Old Trafford.
Because there are players on the staff at the moment with whom United are simply never going to win a major trophy – they aren’t good enough for Manchester United!
But they could earn some brownie points with the supporters by making it look like they care on Sunday afternoon.