United went ahead with a Rooney header after three minutes but West Brom equalised when James Morrison stroked in a superb shot from the edge of the box after 14.
West Brom blew their chance to secure a first win over United since 1984 when Peter Odemwingie sent a 62nd-minute penalty horribly wide and they paid dearly for it when Javier Hernandez came off the bench to head home a Rooney corner 13 minutes later.
The win takes United to 41 points, three clear of Manchester City, who are at home to Blackpool later on SaturdaySunderland, West Brom continue upward progress
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