When it rains in Manchester, it pours.
It was a poor performance from the Red Devils, who still looked shellshocked after their previous home defeat against Spurs.
With the injuries piling up and Moyes admitting he probably will not spend in January, according to Sky Sports, the gloom surrounding United's fortunes increase by the day.
At the end of a tough week for Moyes, here are six things we learned from this FA Cup defeat.
Losing games is one thing, not making opportunities to win a game is another.
Today, as with previous days, Manchester United failed to put their opponents under sufficient pressure and once again paid the ultimate price with a fifth home defeat of the season.
With Moyes' pool reduced due to injuries, United showed how weak their squad has become.
The midfield rarely linked with the strikers, with Alexander Buttner being United's prime source of productivity.
This is not a new problem. United fans will proclaim that this issue dates back several seasons and the slow decline to mediocrity has been masked by trophy success.
However, the evidence is clear. United cannot pass the ball to a high enough standard, and it is a problem that cannot be solved overnight.
A team that has no creativity cannot compete at the highest level.
There are only so many times you can the blame the manager for a player's apparent failings.
For months the fan club of Shinji Kagawa have banged the drum in his favour, pointing to his reduced game time for his distinct lack of sharpness.
Today, we saw exactly why Moyes has not been playing the Japanese international.
On Sunday afternoon, Kagawa attempted to find the spaces in midfield only to play five-yard passes and be on his travels.
His influence on Manchester United is zero.
For a fabulous talent like him, the effort is missing. He looks lifeless as a creative force.
The sumptuous attacking artist that Kagawa was at Borussia Dortmund has disappeared—missing in action since his transfer in 2012.
It's less than two years on and Kagawa is a player without confidence, but more worryingly he looks like a defeated man.
How long Moyes can persist with him is the question at large. The clock is ticking.
Alexander Buttner is a full-back, but the player is far from being a great defender.
On Sunday Buttner showed what he is good at: taking on his opponent on the front foot and whipping in crosses.
With the injuries to Nani and Ashley Young, United are short on wing options.
With Wilfried Zaha not in contention and a heavy reliance on Adnan Januzaj and Antonio Valencia, Moyes could turn to the busy Dutchman, who is much more comfortable in the advanced positions than he is when having to retreat.
The opportunity to play left-wing might revitalise the player's chances this season and give Moyes a much-needed fresh option during this injury crisis.
The Da Silva brothers both have big hearts in their young bodies.
However, as Rafael had time to develop his game with a United shirt on his back, Fabio suffered with injuries and loans.
It now appears his ship has sailed.
In several crazy minutes Fabio entered the fray for United to only be rightfully sent off for an irresponsible lunge.
It is the sort of thing his brother might have done a few years ago, but as third-choice left-back at the club, Fabio has to take his chances to play and not destroy the trust Moyes has in him.
On Sunday afternoon, that trust was obliterated.
The Brazilian may not get another chance this season to shine, and that might put him at the front of the queue for the exit door in the summer.
He was one of United's sole bright sparks in this FA Cup tie.
Darren Fletcher played at the heart of midfield and did his job like the Fletcher of old.
Clearly the player is not the long-term answer for United, but his performance on Sunday showed he might do more than just warm the bench in the coming months.
His pass set United on their way to their only goal of the match, and the Scotsman completed the full 90 minutes.
Fletcher can ease the burden on Michael Carrick as he returns from injury. He can also give United some much-needed leadership, which is so evidently missing in their ranks.
David Moyes looked the loneliest man inside Old Trafford on Sunday.
His team's defeat in a competition that could rescue United's season was only a footnote in this story.
Moyes has been let down by his board and employers. As United fight to keep their heads above water, Moyes has been left in shark-infested waters in a leaking ship.
The failure to strengthen the squad at all costs as the greatest manager in history leaves the club is wholly unforgivable.
Ed Woodward has not covered himself in glory in his new position at Manchester United, and it could be that the loss of David Gill will be more costly to the club than that of Sir Alex Ferguson.
Moyes has struggled with what he has been left with. A banquet is just an empty room when the meal is over.
At the last minute the club purchased a player Moyes knows very well, but it was clear that Marouane Fellaini was not his first choice.
The Glazers now need to reach deep into their pockets and spend.
They need to show the world that their club and their brand are still one of the strongest in the world, and they can only do this by bringing in world-class talent to the team.
Will they do such a thing? Their future success or failure will answer that question very soon.
The Glazers are not gamblers. To leave this squad in its current state would be the biggest gamble they have ever taken.