The 2005 season began with mixed results for Contreras. Contreras started the season with five consecutive no-decisions and an ERA of 3.04, but pitched poorly after that until the All-Star break, with a 4–3 record. The first game after the break (July 14), Contreras pitched extremely well, beating the Indians 1–0. As the White Sox raced to the playoffs in the second half of the 2005 season, Contreras became Chicago's most reliable pitcher, winning his last 8 starts and on occasion halting the team's losing streaks. One of the main reasons was the way he pitched. With the help of fellow Cuban Orlando Hernández, Contreras began dropping his arm angle during his delivery. Contreras also contributed the 3 postseason victories that tied Andy Pettitte for the most second half victories in the Major Leagues with 11, and he threw his first Major League complete game against the Minnesota Twins, on September 23 Contreras finished the season 15–7 with a 3.61 ERA and 154 strikeouts in 204 innings.