On March 25, 2008, Cash's contract was purchased. He acted as the personal catcher for knuckleball pitcher Tim Wakefield after Mirabelli left the Red Sox. Cash made his only postseason appearances as a player in 2008: in the Division Series, he caught two innings in one game without batting, and in the League Championship Series he appeared in three games (one start) and was 1-for-3 at the plate; his one hit was a home run. On December 12, 2008, Cash was non-tendered by the Red Sox, officially making him a free agent.