During the pre-season, Green averaged 8 points and 1.3 rebounds per game for the Celtics in the 2005 Las Vegas Summer League. After seeing limited playing time during the first part of the season, Green was placed on the Fayetteville Patriots of the NBA D-League by the Celtics in January 2006. He was recalled and activated to an NBA roster position on February 3. He was reassigned to the NBDL, to the Florida Flame, on February 16. He was recalled and activated again on February 21, and saw his first significant NBA game action on the following night, scoring 13 points and grabbing 9 rebounds in 23 minutes. He appeared in 32 games and averaged 11.8 minutes, 5.2 points, and 1.2 rebounds during these contests. Green left marks on the league with his highlight plays including a shot clock beating half court three against Indiana and two "SportsCenter" top play dunks in the closing seconds in two games against the Toronto Raptors. Over the off-season, Green aimed at improving all aspects of his game in order to gain playing time. He had shown flashes of his capability during the end of the 2005–06 season, but started off the 2006–07 season slowly, earning a somewhat limited role and playing time behind captain Paul Pierce, and swingman Wally Szczerbiak. With injuries to Szczerbiak and Pierce, his playing time increased, but he was unable to nail down a starting role. Green started his first-ever game early December against the Philadelphia 76ers, but then lost his job to a returning Szczerbiak. Green finished the 2006–07 season averaging 10.4 points, 2.6 rebounds and 1.0 assists per game. On February 17, 2007, Green won the 2007 NBA Slam Dunk Contest, beating out Nate Robinson, Tyrus Thomas, and Dwight Howard. The season was going very well for Green. On March 16, 2007, he scored a career-high 25 points in a Celtics loss to the Dallas Mavericks, and on April 10, 2007, Green topped that with 33 points (12-22 FG, 4-5 3FG) at Atlanta. On July 31, 2007 it was made official that Green was to be traded along with Al Jefferson, Ryan Gomes, Theo Ratliff, Sebastian Telfair, and draft picks, to the Minnesota Timberwolves for Kevin Garnett. At Minnesota, Green found himself stuck on the bench behind a number of players at his same position. In early January 2008, Green's agent asked the Timberwolves to trade his client.