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.
More than 9,000 West Ham fans made their way to Manchester and many more are still reeling from a stonewall penalty on Payet which was turned down by referee Martin Atkinson.
Formerly England's top referee, the opinion of Howard Webb, carries some weight when it comes to crucial, game-changing decisions.
And Webb, who took charge of the World Cup final between Holland and Spain in 2010, told BT Sport it was a penalty.
Top refs wade into Old Trafford controversy
He also waded in on the debate over Manchester United's controversial equaliser, which Webb argued should have been chalked off for a foul on Hammers 'keeper Darren Randolph.
"Penalty," Gallagher told Sky Sports.
"It is difficult for Atkinson in real time, but I think he's Marcos Rojo) caught him.
"The assistant probably had the best view, but whether Atkinson has asked him, we don't know."
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.
Gallagher added: "Foul. He blocks the goalkeeper off.
"There is so much going on there, and I can understand why this is missed.
"To illustrate that, we've had to have it pointed out and watched many times."