The Reds were rampant and Olsson was relieved to see another deflection go behind for a corner when getting in the way of a van Persie chip while Phil Jones' shot looped up for Foster to catch. Hernandez's general play was superb but his finishing touch, unusually, was lacking as he got a free header all wrong when captain Michael Carrick swept the ball across.
On the half-hour mark, Carrick's pass was expertly dummied by Hernandez and Tom Cleverley invited Büttner to bury a low drive into the far corner of the net for a well-worked third. There was dissent from the home fans about the manner of their team's collapse and it seemed to spark their players into life with Shane Long heading over from a rare opening.
However, Hernandez really should have got his name on the scoresheet when an expansive pass by Anderson was volleyed first-time into the Mexican's path by Valencia but the shot lacked accuracy and the Baggies escaped. It had a major bearing on the half-time situation as Morrison, a scorer against United in the past, beat Jones to tuck Graham Dorrans' low cross past Anders Lindegaard for 3-1.
Both sides threatened to score again in the five minutes before the break. Valencia and Cleverley set up Hernandez and, although his shot was straight at Foster, the ball cannoned back goalwards off McAuley to force the keeper into making a stunning tip-over. The home side could, somehow, have ended the half only a single goal in arrears as a Jones lapse let in Morrison but Jonny Evans made an excellent saving challenge as Long looked certain to fire home. Steve Clarke pepped up his attack with the introduction of Chelsea loanee Lukaku for the second half and it gave the home fans some hope of a comeback.