Cash on Tuesday also said rookie Shane Baz will take the mound for Game 2 on Friday night. Depending on bullpen use in the opening two games, Cash said Drew Rasmussen is the "highly likely" Game 3 starter.
Tampa Bay will face the Boston Red Sox in the series.
McClanahan, who last year became the first pitcher to make his major league debut in the postseason, went 10-6 with a 3.43 ERA in 25 starts this season.
Baz, a member of the silver-medal winning U.S. Olympic team, was 2-0 with 2.03 ERA in three starts after being called up from Triple-A Durham last month.
Rasmussen, acquired from Milwaukee in May, went 4-0 and had a 2.44 ERA in 20 games, including 10 starts for Tampa Bay.