Indeed, within five minutes, the Belgium international was allowed to pick the ball up outside the box and roll a softish, but accurate, shot that Lindegaard could only push into the net - much to the Dane's frustration. Yet any thoughts of United ending Sir Alex's reign by relinquishing a three-goal lead looked to initially have been extinguished.
The Reds' response was emphatic with Valencia again to the fore, teeing up Anderson for a shot that Foster saved before leading the charge down the right to flash over another low cross that was placed high into the net by van Persie. It was the Dutchman's 26th league goal for the club and 30th in all competitions from an outstanding debut campaign.
Hernandez saw McAuley snuff out another chance and, after Valencia made an important clearance at the other end, Ryan Giggs came on to join the cavalry. The veteran skipped down the left wing to meet a Kagawa pass as a breathtaking move was capped off by a perfect centre allowing Hernandez to tap in his 50th goal for the club and just reward for his perseverance.
West Brom should have pulled another goal back when Olsson contrived to sky an opportunity from a couple of yards out while Lukaku hammered a free-kick wide and skimmed the side-netting from another promising position. The warning signs were there.
Although a Büttner cross hit the bar and Giggs chipped onto the top of the net, most of the attacking momentum was with the Baggies. However, nobody could have forecast the crazy finale. Dorrans and Billy Jones had efforts before danger-man Lukaku ran onto a Markus Rosenberg through ball to notch his second goal. Within a minute, from the next attack, Youssouf Mulumbu strode through unchallenged to touch in a Jones pass and the comeback was very much on.