Dave Caldwell of the "New York Times" wrote on September 3 that Lieber would "probably end up in the bullpen" in the playoffs, but less than a month later, Mark Feinsand of "MLB.com" wrote that Lieber "has suddenly become one of the most important cogs in the Yankees' postseason machine" due an injury to Kevin Brown and late-season struggles by Vázquez. Lieber started Game 2 of the AL Division Series against the Minnesota Twins. He allowed three runs over innings and was in line for the win, but he received a no-decision in a 12-inning, 7–6 Yankee victory. The Yankees won the series in four games. In Game 2 of the AL Championship Series against the Red Sox, Lieber allowed one run and three hits in seven innings, earning the win in a 3–1 victory. In Game 6, he allowed four runs in innings and took the loss in a 4–2 defeat. The Red Sox become the first MLB team in history to overcome a 3–0 series deficit, winning in seven games. 2004 would be Lieber's only playoff experience, as well as his only stint in the AL.