Green led Montgomery's St. Jude to the Class A State Championship in 2006 and 2008, and was voted the MVP of the 2006 tournament. He scored more than 2,500 points in his high school career, and was a member of the National Honor Society at St. Jude. He also garnered many awards in his high school career, including: 2008 McDonald's All-American, 2008 Alabama Mr. Basketball, a three-time Alabama Sportswriters Association "Player of the Year" (2006, 2007 and 2008), and was also rated the #6 overall and #1 power forward in the 2008 recruiting class according to ESPN. Green also played for the USA National U18 Team in Argentina at the 2008 FIBA Americas Under-18 Championship where he was coached by Anthony Grant, who would eventually serve as his head coach during his time at Alabama.

On August 23, 2007, Green committed to Alabama over Ole Miss. During his freshman year at The Capstone, Green started all but one of 32 games on the season, while averaging 11.6 points per game and ranking first in rebounding among SEC freshmen with 7.9 per game. He was second among all SEC players with five double-doubles, including four consecutive double-doubles against Georgia, Vanderbilt, Florida and Mississippi State. He was a unanimous Freshman All-SEC selection, and named a collegeinsider.com Freshman All-American.

During his second season, he finished ninth in the SEC in rebounding (7.2) and 17th in scoring (14.1). He scored in double figures in 10 of the last 11 games in the 2009–10 season, including eight straight at one point. Green was named to the 2009 Old Spice Classic all-tournament team; he won the game against No. 15 Michigan with a put-back dunk with 5.4 seconds left and blocked Michigan's game-tying shot attempt as time expired. Green posted his career-high 27 points with 13 rebounds in a win over Mercer and was named the SEC Player of the Week for his efforts.

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