When Kevin Friend, the fourth official, held up his board to reveal four minutes of added time at the end of this compelling tie, a loud groan echoed round the Boleyn Ground.
Once again in this season of late, late finishes, it was Robin van Persie who made the difference for United. Introduced as a substitute for Javier Hernandez, the Dutchman scored an equaliser of class and precision to rescue his side from elimination from the Cup.
As the 90 minutes came to an end, Van Persie undid West Ham with but three touches. Running on to a sublime 60-yard pass by the other substitute Ryan Giggs, his first took him away from James Tomkins, his second advanced him further into the West Ham area and his third arrowed the ball past Jussi Jaaskelainen into the bottom of the goal.
Van Persie's response, nodding in self-appreciation, suggested he was as unsurprised by his defining contribution as the thousands of relieved United fans packing the Trevor Brooking stand. His strike even had the opposition manager purring.
"It was so silky and smooth," said Sam Allardyce, who added that at the rate the player is scoring, the £24m he cost from Arsenal can already be reckoned the steal of the season.
"Eighteen goals in whatever answers that. The man himself picked the team where he thinks he can get the most trophies, not make the most money, and that is to his credit."
Magnanimous as he was, Allardyce was aware of the irony in seeing his team undone by a long ball.
"United don't hit it long do they," he said. "How long was that one?"
Though he might not take it as such, it was something of a compliment that it had required the direct combination of two of United's substitutes to salvage a draw against his team.
Until the cavalry had been summoned, several of the visitors' second string had done themselves few favours in their attempts to force more regular selection.
Friend could have added four hours to the play and it is unlikely Shinji Kagawa, for instance, would have had any influence over the direction of play.
With Kagawa anonymous to the point of invisibility, after Tom Cleverley had expertly converted Hernandez's shrewd pass midway through the first half to put United ahead, West Ham were able to seize the initiative.
They exploited the aerial weakness that has dogged United all season.
Twice in almost identical circumstances, centre-half James Collins rose unchallenged to head past David De Gea.
Both crosses had come from the same source – Joe Cole.
In other circumstances, if say Van Persie had remained on the bench or in north London, the narrative of this match would have centred around the prodigal's return.
From the moment the stadium announcer had asked the locals to "please welcome home No 26 Joe Cole," the player seemed keen to reacquaint himself with every blade of grass at the stadium where his career began.
This was a man clearly anxious to remind us of the prospect he once was in these parts. He may have been £12m richer from his nine league starts at Liverpool, but the frustration at his neglect on Merseyside was evident in his every dash for the ball.
When he got it, he used it well, causing havoc in United's backline with his crosses. The locals responded to his effort.
"It was amazing, it choked me up," he said of the ovation that greeted his substitution after bloodying his nose in the claret cause. His manager added such reaction might just be the spur to reproduce the form that had once made him the object of Chelsea, Liverpool and England's desire.
"The fans made him feel good," said Allardyce.
"It made him feel good when he ran on – how they welcomed him and more importantly, even better when he came off, even bigger applause for what he's done."
With Cole providing such vibrant delivery, for a few minutes the boisterous West Ham fans were dreaming it was 2001 all over again, reminding themselves of FA Cup victory inspired by Paolo Di Canio at Old Trafford.
But then Van Persie stepped off the bench to intervene.
As everyone inside the Boleyn knew he would. (© Daily Telegraph, London)