In 2004–05, he struggled to break into Milan first team, making only made 22 Serie A appearances and scoring one goal. It is his late goal, however, that took Milan to the UEFA Champions League final. With Milan losing 2–0, a scoreline which would have sent the match to extra time as Milan defeated PSV with the same score in the San Siro, Ambrosini scored a header in second half injury time to bring the score to 2–1 and give Milan the 3–2 aggregate lead. PSV immediately responded with a goal of their own to tie the aggregate score at 3–3, but Milan advanced to the final on the away goals rule. Ambrosini, however, missed the final in Istanbul with yet another injury as Milan were defeated on penalties by Liverpool.