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 with seven minutes left to ensure a rematch will be needed at Upton Park to decide who will face Everton at Wembley in the semi-final.
There has been a lot of talk that the replay is a blessing in disguise for the Hammers.
Yes it sets up a fitting last ever FA Cup game at the club's beloved Boleyn Ground home of 112 years ahead of their big move to the Olympic Stadium this summer.
A last ever FA Cup game under the lights will bring the best out in the West Ham fans who will create a real hotbed atmosphere as they did against Tottenham Hotspur recently.
They will undoubtedly help the team and push the players to give everything to seal a return to Wembley for the first time in four years since West Ham's play-off final victory over Blackpool in 2012.
It will also give Bilic the chance to get key defenders Winston Reid, James Collins and James Tomkins back fit.
That will in turn allow the former Croatia boss to push Michail Antonio back to his natural attacking position and bolster his midfield options.
Could work in Red Devils' favour
But the replay could swing in Manchester United's favour more.
If, as is expected, Louis van Gaal's side are eliminated from the Europa League by bitter rivals Liverpool this week the FA Cup will represent the club's only chance of silverware this season.
Europa League elimination will also free up their gruelling schedule.
Most importantly however, it also means key playmaker Juan Mata will be eligible to play while Wayne Rooney could also return from injury in time and he has scored more goals against West Ham than any club.
Remember before now the last ever FA Cup game at Upton Park was that unforgettable last gasp extra-time win over Liverpool in the fourth round and whatever the result against Manchester United it will replace that game in the history books forever more.
It is an apt last ever game for the famous stadium, but Hammers fans will be hoping it gets a fitting send off and they are not left feeling their chance had gone at Old Trafford.