Lionard Pajoy also scored for the Union (6-9-2), who won their third in a row and fourth in five games.
Five minutes of stoppage time was added and Valdes didn’t take long before scoring the game-winner on an assist by Sheanon Williams.
Following a 2-0 loss to Sporting Kansas City last Wednesday in the semifinals of the Lamar Hunt U.S. Open Cup, the Union came out sluggish for much of the game against the Impact.
But the speedy Hoppenot, a second-half substitute, ignited Philadelphia and helped secure the victory.
The Impact have lost four of five and fell to 6-12-3.
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