In his first game of his 2006 campaign, Allen recorded 9 points, 7 rebounds, and 2 steals during a home game loss to the Hawks. On March 20, 2006, Allen recorded a season-high 18 points and 4 steals in a home loss to the Los Angeles Lakers. He tied his season-high 18 points on April 9, which came in a loss to the New York Knicks. Eight days later, he broke his season-high as he scored 23 points to go along with 4 rebounds and 3 steals in a home game loss to the Cleveland Cavaliers. Allen went on to average 7.2 points, 2.2 rebounds, 1.3 assists and 1.00 steals in 19.2 minutes in 51 games (nine starts) in his sophomore season with the Celtics.