Erik ten Hag insisted his side would be able to cope without Casemiro, but the truth is that they barely did against Fulham in their FA Cup quarter-final win.
United were always going to be affected by the absence of arguably their two most important players in Raphael Varane and Casemiro, but the drop-off in quality was glaringly enormous.
Fulham started the match brightly with a flurry of corners and only grew in confidence when it became apparent that United simply were there to be beaten in midfield.
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There were a few impressive individual moments defensively to keep their side level early on, but this very much felt like a team of individuals assembled to put a plaster over the damage done to their line up.
Fulham always looked like a threat, even if they proved to be lacking in the final third, with Joao Palhinha forming the foundation for their impressive gameplan to function.
The 27-year-old has arguably been Fulham's best player on their impressive return to the top flight, and he could well be playing back in Europe next season, even if his current club side aren't.
Palhinha had impressed in the first meeting between the two sides last November and relished the opportunity to assert his dominance against a United side with a weakened spine.
United were one of the sides loosely linked with the forming Sporting star last week, and although a move for him seems highly unlikely this summer, another player in his mould is certainly required.
Ten Hag has been unfortunate that his side's reliance on Casemiro has been exploited so often this season, and they will hope his suspension issues are a freak occurrence rather than form going forward.
However, if they want to challenge on all fronts in the final months of the season and next season as well, then it is essential they have a proper alternative to their star midfielder, rather than an awkward makeshift solution.
Scott McTominay and Marcel Sabitzer both have some merit as squad players in this United side, but they are too similar to start together and struggled to offer any control in the middle of the park.
They are both attack-minded midfielders who like to make driving forward runs, leaving United light in central areas and without any defensive cover whenever they lose the ball.
McTominay has shown an enthusiasm to fill in the deeper role whenever it has been asked of him, and although he has many of the physical traits needed to occupy the position, he lacks creativity and the passing ability that Casemiro offers.
This was confounded when McTominay was replaced by Antony less than an hour into the match, with Ten Hag instead preferring to drop Fernandes into a deeper role and use Wout Weghorst in attacking midfield.
Fulham's reckless red card implosion gifted United a way back into the game, and as soon as Fernandes netted from the spot, it seemed certain that a second would follow. Sabitzer, now with licence to push forward as much as he wanted, did just that and finished brilliantly to all but settle the tie.
There was little room for pessimism even after a largely poor performance, but this was another match in which United somehow managed to win when they weren't at their best.
Ten Hag will argue that his side did manage to win without Casemiro again, though this victory was owed as much to Fulham's own wild decision-making as it was about their own performance on the day.
United have already outlined plans to sign an elite striker and another creative midfielder this summer. They need another defensive anchor too.
Palhinha might want to see red differently.
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