After Wayne Rooney had given United an early lead, De Gea dived over Shane Long’s shot and allowed the ball to squirm under his body into the net before half time.
The 20-year-old De Gea was signed in the offseason to replace retiring Dutch goalkeeper Edwin van der Sar.
His mistake, which followed errors in the Community Shield last Sunday, did not prove to be costly as Ashley Young’s late cross was deflected into the net by West Brom defender Steven Reid.
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