 On July 7, 2008, Masterson was sent back to the Pawtucket Red Sox, a move manager Terry Francona stated was made to transition Masterson from a starter to a relief pitcher.Masterson was recalled on July 20 due to an injury to David Aardsma In his first relief appearance, he was solid against the Seattle Mariners, shutting down the hitters and working scoreless innings at Safeco Field in Seattle.Masterson picked up the first postseason win of his career in Game Five of the 2008 American League Championship Series (ALCS) against the Tampa Bay Rays, pitching a scoreless ninth inning in the Red Sox' 8–7 victory.