As a member of the Twins, Smithson led the American League in games started in and . He won 15 games in each season. The Red Sox brought Smithson back as a free agent in , and he spent two seasons with them as a swing man, making 37 starts in 71 games. Along the way, he pitched against the Oakland Athletics in the 1988 American League Championship Series, his only postseason appearance, providing 2 innings of scoreless relief in Game 4, which Oakland won to complete a sweep over the Red Sox.