Manager Dusty Baker announced the decision Tuesday but did not reveal his rotation past Game 2. He previously said Lance McCullers Jr. will start in the series opener Thursday.
Valdez, 27, won a career-high 11 games this season with 125 strikeouts, and his 3.14 ERA in 22 starts was his best since his rookie season in 2018. He missed the first 49 games this season after breaking his left index finger in his first start of spring training.
Valdez made his postseason debut last year and was 3-1 with a 1.88 ERA in four games with three starts.