Undrafted, Shannon was signed as a free agent by the Mighty Ducks of Anaheim on April 3, 2005. He played for their American Hockey League (AHL) affiliate, the Portland Pirates, in 2005–06 and immediately began an eight-game scoring streak to begin the season. Midway through the season, he participated in the 2006 AHL All-Star Classic for Planet USA and won the fastest skater competition. Finishing his AHL rookie campaign with 86 points in 71 games — ninth overall in league scoring and second among rookies to Patrick O'Sullivan — he was named to the All-Rookie Team. Shannon added 22 points in 19 playoff games as Portland was eliminated in the seventh game of the Calder Cup semi-finals.