Sir Alex's side's neat attacking play had warranted a first-half lead through Tom Cleverley, but while United tried to rule with possession West Ham commanded with power and presence, as Joe Cole twice assisted headers either side of half time for towering Hammers centre-back James Collins.
The tie looked over, then two substitutes combined to earn a replay. Ryan Giggs, who seconds earlier had missed a golden opportunity to head United level, produced a sublime pass that was received with equal aplomb by van Persie and, once his sights were trained on goal, his stoppage-time finish inevitably ended in only one outcome.
The Reds have had mixed fortunes in the cups against West Ham in recent times. The often-referenced fourth round defeat in January 2001, in which Paolo Di Canio scored the winning goal as Fabian Barthez stood arm aloft claiming offside, was an obvious disappointment. But two years later United exacted revenge with a 6-0 hammering at Old Trafford – Ryan Giggs and Ruud van Nistelrooy grabbed two goals each, with Phil Neville and Ole Gunnar Solskjaer also in on the act.
The most recent knockout competition meeting was in the Carling Cup in November 2010, in which West Ham hit four goals past a much-changed Reds side. Jonny Evans, Chris Smalling and Javier Hernandez were all involved that night and were named in United’s side for this visit to Upton Park.