As the rain started to cascade down again, the match remained in the balance with Foster barely tested until clawing away a Valencia attempt seconds before Kagawa made way for top scorer van Persie. Clarke made a change of his own by introducing Chelsea loanee Romelu Lukaku to the action and the substitutions added punch to both attacks.
There was a let-off when McAuley almost atoned for his own goal by heading against the bar but referee Jon Moss rightly awarded a free-kick as the defender was climbing over Smalling to get to the corner. Nonetheless, it was another warning, if any was needed, that the high-flying Baggies were capable of getting something out of the encounter.
Sir Alex restructured the side in a bid to hold onto the lead with Valencia dropping to right-back and Smalling switching to the middle to form a five-man defence. It left United short of options going forward as the crowd became increasingly nervous, with this tension transmitting itself to the players.
A chance was finally carved out when Evra threaded the ball through to van Persie but the Dutchman's shot was saved by the legs of Foster. Vidic, excellent throughout, unconvincingly helped an Odemwingie drive out for a corner as the pressure continued with the leaders only capable of occasional breakaways led by the isolated van Persie. Yet the Reds' leading scorer still managed to have the final say after Welbeck's shot had forced a corner through Foster's diving stop.
Subsitute Paul Scholes had a shot blocked before helping it on to van Persie via Valencia and the deadly striker did the rest, biding his time before bending a brilliant effort over Foster to double the advantage and finally sink the brave Baggies.