There was to be no rest bite for the Baggies defence as the United onslaught continued, with Welbeck firing inexplicably wide with just Foster to beat.
However, any chances of an Albion fightback were relinquished as the visitors pressed the self-destruct button shortly after the hour mark.
First of all, Jonas Olsson crashed into the back Hernandez who was just too fast for the Swedish defender, and having been booked just 15 minutes earlier, the Albion man was heading for an early bath.
Rooney stepped up and coolly slotted home his 20th league goal of the season, sending Foster the wrong way.
Roy Hodgson introduced the energetic Shane Long with 20 minutes to go, and the Irishman almost made an immediate impact, forcing de Gea into a smart save at his front post.
The fightback never really looked like materialising from Roy Hodson’s men, as United saw out a comfortable victory, moving to the top of the table at the expense of their Manchester rivals, West Brom stay in 13th.
