Chacín began with the "AA" Eastern League New Hampshire Fisher Cats. He was 16–2 with 109 strikeouts and a 2.86 ERA in 1412/3 innings, having earned his promotion to the "AAA" International League Syracuse Chiefs after a 34-innings scoreless streak. With the Chiefs, Chacín posted a 2–0 record with 14 strikeouts and a 2.31 ERA in 11 2/3 innings. Then, he was promoted to the Blue Jays and made his debut on September 20, 2004 at Yankee Stadium. Chacín allowed two earned runs on three hits over seven innings for the win, as Toronto beat the Yankees 6–3. In his second start, Chacín held the Baltimore Orioles to four hits and a run in seven innings, but took the loss. At the end of the season, he had compiled a 1–1 record with six strikeouts and a 2.57 ERA in 14 innings