First-half goals from Javier Hernandez and the returning Wayne Rooney, back in the starting XI amid the furore over his future in the wake of his omission against Real Madrid, seemed set to secure a comfortable victory at Old Trafford.
Former Liverpool boss Rafa Benitez was being tormented by United fans with chants of, 'you're being sacked in the morning' but the under-fire Chelsea interim boss earned some respite thanks to Eden Hazard and then Ramires' equaliser.
The replay will not suit Ferguson, still less so Benitez given it could potentially force Chelsea into four games in a week or trigger the postponement of a Premier League fixture.
However, both have reason to be thankful, United because their collapse from a position of such immense promise could so easily have been total, Chelsea as at half-time it seemed Benitez was being given another hefty shove towards the Stamford Bridge exit door.
It had all been about Rooney as the game kicked-off but Hernandez had already struck in quite spectacular fashion before his team-mate found the net, heading Michael Carrick's perfect pass past Petr Cech in the fifth minute.
Rooney made it two just six minutes later, lining up a free-kick wide on the United flank, level with the penalty area, he aimed for the far corner and Cech could not react quickly enough.
Rooney nearly got a second in first-half stoppage time, as he deliberately sent another free-kick from a similar position on the other side of the pitch fizzing on to the roof of the net.
The vitriol aimed at Benitez from the away contingent to their own interim boss was even worse when he took Frank Lampard off as part of a double change nine minutes after half-time.
But Benitez is not quite the know-nothing Chelsea's disgruntled supporters think and his tactical switch worked a treat as one of the men introduced, Hazard, injected fresh hope into the Blues by curling a superb effort into the far corner beyond David de Gea on 59 minutes.
The impetus it gave Chelsea was marked, and when Rooney lost possession deep in the visitors' half, United were caught out with a classic counter-attack that ended with Ramires expertly drilling home in the 68th minute.
It was the second time this season the Brazilian had completed a Chelsea comeback from two goals down against United and Juan Mata very nearly one the game this time but was denied by De Gea in stoppage time.