Davies scored his second goal for the club with a thunderous volley on 22 minutes and Hernandez wrapped up the visitors’ first win of the season with a close range finish on 83 minutes.
And Swansea manager Michael Laudrup described his team’s performance as “perfect”.
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“An away win, a clean sheet and the way we did it was perfect,” he said. “I was thinking ‘are they going to equalise?’ but fortunately they didn’t and we won the game.”
West Brom manager Steve Clarke, whose team are yet to score in three Premier League games this season, hopes to strengthen his squad before tonight’s 11pm transfer deadline.
He said: “I need some players in forward areas that can give us a bit of quality so we can create chances.”