With the score tied 1-1, Matt Downs struck a two-out, pinch-hit double against Sean Marshall (4-5), Matt Dominguez was intentionally walked and pinch-hitter Brandon Barnes was hit by a pitch to load the bases.
That brought up Jose Altuve, who sent a seemingly routine grounder to second base. But Valdez -- who was brought in as a defensive replacement -- let it roll between his legs and Downs came home with the game-winner.
The rally handed a victory to Wilton Lopez (6-3), who got the final two outs in the top of the ninth as the Astros snapped a six-game losing streak.
Cincinnati starter Homer Bailey allowed only one run on three hits over eight innings.
Ryan Ludwick supplied the only RBI for the National League Central-leading Reds.