After completing his collegiate career, Liles signed a two-year, entry-level contract with the Avalanche on March 29, 2003. Signing an amateur try-out contract the following day with Colorado's American Hockey League (AHL) affiliate, the Hershey Bears, Liles made his professional debut against the Grand Rapids Griffins on March 30 and played in ten games to end the 2002–03 season. In the 2003–04 season, Liles made the Avalanche roster out of college and scored his first NHL goal and point, a game-winner, against the Minnesota Wild on October 16, 2003. He led all rookie defensemen with ten goals and 34 points in 79 games to be named to the NHL All-Rookie Team while also becoming highest point-scoring rookie defenseman in Avalanche history, and second in franchise history since Bruce Bell's 37 with the Quebec Nordiques in 1984–85. During the NHL lockout the next season, Liles enjoyed a brief stint in the German Deutsche Eishockey Liga (DEL) for the Iserlohn Roosters, playing in 17 contests and scoring 11 points. He then returned to the Avalanche for the 2005–06 season and established himself within the Avalanche defensive corps, appearing in all 82 regular season games and finishing tied in first in goals scored by a defenseman with Rob Blake, with 14. Improving his offensive numbers in every category, Liles also set a new team record among defensemen with 14 points scored in the month of October. Following another impressive season, he then signed a two-year contract extension on July 12, 2006. With the arrival of rookie Paul Stastny surprisingly making the team in 2006–07, Liles relinquished his number 26 jersey, worn by former Quebec Nordiques great Peter Šťastný, for Paul Stastny to wear in honor of his father. With the change to the number 4 jersey, Liles led the Avalanche defense in scoring with 14 goals and 44 points, placing tenth in the NHL. In reaching his tenth goal against the Dallas Stars on December 27, 2006, Liles became the first defensemen in the NHL since Steve Duchesne in 1987–88 to score at least ten goals in his first three NHL seasons. After suffering the disappointment of failing to qualify for the Stanley Cup playoffs for the first time in Avalanche History, Liles helped lead the team return to the post-season in 2007–08. On December 17, 2007, he recorded two helpers against the Los Angeles Kings to surpass 100 career assists in the