Born in Borgo Valsugana, Trentino, Stefani started his professional career with A.C. Milan of Lombardy, at first as a midfielder. He played his first and only match for A.C. Milan first team on 24 May 2003, the last Serie A match of the season and the match before 2003 UEFA Champions League Final and 2003 Coppa Italia Final. He was substituted at halftime with Mattia Dal Bello, Milan already losing 1–3 at that time.