Abate started playing football with amateur club Rescaldina, before joining AC Milan's youth system in 1999. During the 2003–04 season, he made his professional debut, playing in a Coppa Italia game against Sampdoria on 3 December. He also made his debut in European competitions, in a Champions League group-stage game versus Celta Vigo on 9 December. In that match he set the record as the youngest Milan player to ever play in the UEFA Champions League, at 17 years and 27 days.