The Hammers were desperately unlucky to be held to a 1-1 draw by the Red Devils on Sunday after Dimitri Payet's sumptuous late free-kick looked to have given Slaven Bilic's men a deserved place in the semi-finals at Wembley.
But Anthony Martial bundled home a scrappy equaliser to set up a fitting last ever FA Cup game at West Ham's Boleyn Ground home of 112 years ahead of their big move to the Olympic Stadium this summer.
The East Londoners are still reeling after seeing a stonewall penalty on Payet turned down by referee Martin Atkinson while he also pulled the Irons hero up for a foul that wasn't when clean through on goal.
Game littered with controversy
And that was not the only controversy.
West Ham were also understandably furious with the officials for failing to spot an obvious foul on their goalkeeper which allowed Martial to bundle home at the far post with seven minutes remaining.
But could the blame also lay a bit closer to home?
Did Carroll decision cost Hammers dear?
When Andy Carroll was named in the starting line-up ahead of Diafra Sakho it raised a few eyebrows among Hammers fans.
However, the big striker put in a fantastic all action performance to fully justify his inclusion.
While he was a constant threat up front he also helped repel a number of United attacks and set pieces by winning crucial headers in the box.
When Bilic elected to bring Carroll off late on it took many by surprise given West Ham were leading 1-0 and only had to hold on for another 15 minutes to book their date at Wembley.
Bilic has been brilliant for West Ham this season but it turned out to be a decision which ultimately cost his side dear.
That's because when the cross came over for Martial it was Sakho who was beaten at the far post as he missed a header to clear the danger, a cross Carroll had been helping mop up all game long.
The sides will now go at it again back at what promises to be a raucous Upton Park in the replay to decide who plays Everton for a place in the final next month.