Boston Celtics (2005–2007)
Green was expected by many analysts to be one of the top players chosen but he fell to the Celtics, who selected him with the 18th pick in the 2005 NBA draft. His fall is attributed to his decision to give individual workouts only for the teams with the top six picks prior to the draft.
Green was one of the last players to enter the NBA directly from high school, as the collective bargaining agreement between NBA owners and the National Basketball Players Association now mandates that American players who enter the draft must be at least one year removed from the graduation of their high school class and reach age 19 no later than December 31 of the calendar year of the draft.
During the preseason, 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.