BOSTON (AP) — The Celtics have obtained forward JaJuan Johnson of Purdue, taken by the New Jersey Nets with the 27th pick of Thursday’s NBA draft, for guard Marshon Brooks of Providence, who Boston had taken with the 25th choice.

The Celtics also get a second-round pick in the 2014 draft. Boston went into the draft needing size and shooting. The 6-foot-10 Johnson, a member of The Associated Press first-team All-America squad this year, provides the size. Brooks, the nation’s second leading scorer with 24.6 points per game this season, would have brought the shooting. Both are seniors.

Johnson, the Big Ten player of the year and defensive player of the year in 2010-11, averaged 20.5 points, 8.2 rebounds and 2.3 blocks last season. For his four years at Purdue, he averaged 13.7 points and 6.1 rebounds and sank 50.1 percent of his shots.

“The sky is the limit for JaJuan,” Purdue head coach Matt Painter said. “This is a great day for him and his family, and it’s the beginning of a great career. He’s a relentless worker who will do a great job of getting himself ready to play.” Johnson is the first player from Purdue chosen in the first round since Glenn Robinson was the first overall pick in 1994.

Celtics general manager Danny Ainge has said he didn’t expect much help from the draft in the coming season, but was hopeful the players he picked — Boston also had the 55th choice — could play some role as rookies and develop into bigger contributors as the team moves on from the Big Three era.

Kevin Garnett and Ray Allen, both 35, are signed for just one more season. Paul Pierce, 33, has three years left on his contract. The only other Celtics signed for next season are guards Rajon Rondo and Avery Bradley, and center Jermaine O’Neal. Coach Doc Rivers is signed for five more years.

The 6-foot-5 Brooks set a single-game Big East record with 52 points against ninth-ranked Notre Dame when he made 20 of 28 field-goal attempts. The 52 points tied Jimmer Fredette of Brigham Young for most in one game last season. Brooks scored at least 20 points in 26 of his 32 games as a senior, and sank 48.3 percent of his shots.

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