Manager Pep Guardiola has warned four key stars could miss the game, with Ruben Dias, Manuel Akanji, Kevin De Bruyne and Jack Grealish all doubtful for Saturday's clash.
The influential quarter did not even make the squad for City's final Premier League game of the season on Sunday, in which a rotated side lost 1-0 against Brentford.
But Guardiola may be forced to rely on some of his squad players against Erik ten Hag's side at Wembley Stadium as they chase a Treble - with the Champions League final against Inter Milan a week after the FA Cup showpiece.
And below, Mail Sport looks at how City could line up if all four players are ruled out.
Manchester City (3-2-4-1)
GOALKEEPER - Ederson
Though Stefan Ortega has played every FA Cup match so far, and lined up in 13 games in all competitions, Ederson is the clear first choice in goal.
Guardiola's sense of fairness and consistency of opportunity may allow the German to start.
But our hunch is that with the Treble on the line he will turn to his trusted No 1.
City lost 3-2 against Liverpool in last year's semi-final after a poor display from then-deputy Zack Steffen.
Guardiola may not want to make the same mistake again.
RIGHT CENTRE BACK - Kyle Walker
Even at 33, Kyle Walker is at the very top of his game. He has not lost any of his searing pace and has even argued that he is improving from a tactical standpoint, rather than declining.
If Dias and Akanji were both fit, he may have missed out, given his playmaking ability from deep is not as accomplished, but he is a lock in the starting XI at the moment and he will likely be used to try to neutralise United's key attacker Marcus Rashford.
CENTRE BACK - Aymeric Laporte
Potentially the biggest winner if Dias and Akanji are ruled out is Aymeric Laporte. The Spaniard was fundamental last season, playing 44 times, but has only played exactly half as many this campaign.
Guardiola's switch to a back three from a back four has cut down his minutes, as has Akanji's emergence and the consistency of the other defensive options.
He may well leave this summer - but Pep can count on him for the final if he is required.
LEFT CENTRE BACK - Nathan Ake
Obviously De Bruyne and Haaland and Stones have been great. But Ake has been one of City's best players this season. Definitely the most improved.
After two seasons of acting as cover only, he has cemented the left-centre back role in this innovative 3-2-4-1 system and can cover left back and defensive midfield too if the shape changes.
Mobile, tactically excellent and with superb anticipation, Ake is one of the first names on the team-sheet.
DEFENSIVE MIDFIELD - John Stones
John Stones will start. The question is, where? If everyone were fully fit, he would play in defence, possibly ahead of Kyle Walker at right-centre back.
But given the defensive injuries, and with De Bruyne also doubtful in attacking midfield - meaning Ilkay Gundogan and Bernardo Silva could fill in further forward - he may well line up alongside Rodri in defensive midfield.
Kalvin Phillips is seemingly not yet fully trusted by Pep and Stones' playmaking ability outstrips the likes of defenders Walker and Sergio Gomez. Could this role belong to teenager Rico Lewis next season?
DEFENSIVE MIDFIELD - Rodri
What a player Rodri is.
United's Casemiro has had a transformative impact on City's final opponents, sure, but the fact the Spaniard has been omitted from so many 'Team of the Season' line-ups is ridiculous.
He is one of the best players in the world, utterly complete in all facets of the game. Put some respect on his name.
RIGHT WING - Riyad Mahrez
Another whose place may have been under threat if everyone were fit is Riyad Mahrez.
Bernardo Silva may be used on the right, for example, if Kevin De Bruyne recovers for the Champions League final.
But at the moment, the right wing slot belongs to the silky Algerian.
ATTACKING MIDFIELD - Ilkay Gundogan
City would be losing a key part of their team if the German Ilkay Gundogan leaves this summer on a free transfer when his contract expires.
He is their captain, set piece taker, a man for the big moments, and his run of goals late in the season helped keep the momentum of their title charge up.
He may yet play alongside Rodri in the double pivot, but can contribute further forward too.
ATTACKING MIDFIELD - Bernardo Silva
Another star who City would do well not to lose in the upcoming transfer window.
Doubtless part of the club's strongest XI when everyone is fit, he can arguably fit in anywhere in the front seven of Guardiola's 3-2-4-1.
Don't bet against him scoring a crucial goal on Saturday.
LEFT WING - Phil Foden
Perhaps the biggest embodiment of City's embarrassment of riches is the fact Foden has often not been a starter this season.
With £100m star Jack Grealish seeming to cement the left-wing slot this campaign, Foden has often been left out in the cold.
But with Grealish a doubt, Foden can step in - and he's already shown how lethal he can be against United, scoring a hat-trick in a 6-3 win at the Etihad in October.
STRIKER - Erling Haaland
He's scored 52 goals in 51 games. In his first season. Enough said.
BENCH - Ortega, Carson, Lewis, Gomez, Phillips, Perrone, Charles, Palmer, Alvarez.
The nine-man substitutes' bench is likely to comprise the backup goalkeeper Ortega, plus third-choice Scott Carson too, who has made the bench on numerous occasions this season.
With seven spots left, they will probably be filled by the only other seven players in the senior first-team squad - though young centre Fin Burns or forward Ben Knight may be included.
Defenders Rico Lewis and Sergio Gomez offer versatile options, while Kalvin Phillips, Maximo Perrone, and Shea Charles - who came on as a sub to make his debut in the final league game of the season against Brentford - provide central midfield cover.
In attack, Guardiola is likely to be limited to winger Cole Palmer, plus World Cup winner Julian Alvarez, potentially the only player with game-changing ability among the likely substitutes.
The fact Carson, Perrone and Charles have only made three appearances in all competitions between them this season shows how threadbare City's squad is looking for the weekend if all four stars currently doubtful are ruled out.Read more 2023-05-30T11:47:18Z dg43tfdfdgfd