The Hammers were desperately unlucky to be held to a 1-1 draw by the Red Devils earlier this month after Dimitri Payet's sumptuous late free-kick looked to have given Slaven Bilic's men a deserved place in the semi-finals.
But Anthony Martial bundled home a scrappy equaliser with seven minutes left to ensure a rematch will be needed at Upton Park On April 6th to decide who will face Everton at Wembley.
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.
The pros and cons have been the subject of much debate.
'Still all to play for'
And the club's co-owner Sullivan has told the club's official website he has been unsettled by what he has seen since that game.
"We have it all to play for and we are still in the FA Cup, though Manchester United are rolling into form at the wrong time for us!" Sullivan told whufc.com.
"However, we fear nobody at the Boleyn Ground having been unbeaten since August."
Reds looking Devilish again
Sullivan makes two salient points.
They looked a potent force with Martial, Jesse Lingard and youngster Marcus Rashford as a front three in their victory over Manchester City last time out.
The Old Trafford side also no longer have the distraction of the Europa League and could have star man Wayne Rooney back for the replay.
But like Sullivan says, the Hammers are unbeaten at home since early slip ups to Bournemouth and Leicester City at the beginning of the season.
And with a partisan Boleyn Ground support behind them West Ham will certainly be no pushovers for the visitors.