If the dropped points in midweek were bad enough, Manchester United's night was made even worse by Casemiro's yellow card.
There were visible grimaces on the faces of the United bench when the Brazilian was booked for a reckless challenge on Wilfried Zaha, pains that would be even harder felt when Michael Olise emphatically punished them with a stunning free-kick.
In the closing moments of the match at Selhurst Park, it was impossible not to get distracted by the prospect of facing Arsenal this weekend, weakened without their star man in midfield.
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Out of all of Erik ten Hag's summer signings, none have made their presence felt on the pitch quite like Casemiro. The 30-year-old has played a vital role with his defensive stability, but more so through his ability on the ball, offering moments of calm amid the chaos and carving opposition defences open with his surgical precision.
Casemiro's expertise as a defensive midfielder is so great that he also knows the vulnerability of the role and uses his insider knowledge as a way to unlock the opposition by preying on their weaknesses.
With such a glowing endorsement, it is easy to see why the former Real Madrid man will be a loss to United this weekend, though his absence might not be the disaster some are predicting.
Ten Hag is right to point out that United have already beaten Arsenal once without Casemiro this season, with the Brazil international starting on the bench in September before being brought on in the final ten minutes of the match.
Marcus Rashford had already scored twice at that point to give United a 3-1 lead in the game, as United once again showcased their brilliance on the break to tear apart their opponents.
While they will undoubtedly miss Casemiro's experience and passing ability tomorrow, his absence will at least provide Ten Hag's team with a clear identity of how they will approach the match.
Without the aid of their star midfielder, it seems implausible that United will go to the Emirates and dictate possession and instead will be better suited to embracing the role of the underdog, as they already have against Arsenal, Liverpool and City this season.
Given that the onus will be on Arsenal to dominate proceedings and have most of the ball in possession tomorrow, it might not be such a problem if United are without Casemiro.
For such an approach, there is less reliance on having a designated defensive midfield specialist anyway, given that the tactic is focused on energy and enterprise to press opponents and quick transitions from defence to attack.
It means that United might well get away with utilising the often derided 'McFred' axis to harass the Arsenal midfield, playing to their strength as industrious workers in midfield rather than exploiting their shortcomings when they are expected to break down opponents.
After a stellar showing against City a week ago, it seems certain Fred will reprise his role in the heart of midfield, though in the home meeting between the two sides, it was a pairing of Scott McTominay and Christian Eriksen that proved successful.
What is crucial is that whoever Ten Hag selects tomorrow shows the same discipline as last weekend in knowing when to engage with the press and forming the backbone of a side that can then counter-attack whenever they get the chance.
Casemiro is a huge loss, but it may not have a significant impact on how United play.
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