With Perugia relegated again, Kalac joined A.C. Milan in the summer of 2005 on a free transfer. It was not until February 2006 that he made his debut in a Milan shirt, when an ankle injury suffered by incumbent Dida forced a substitution in the first leg of the Champions League knockout stage against Bayern Munich. As a result, he played his first Serie A match since 2004, keeping clean sheets against Palermo and Empoli F.C.. He also played in the Coppa Italia tournament until Milan were eliminated by underdog Palermo on a disappointing 3–1 aggregate.