The Boston Celtics have signed the 33rd overall pick in the 2015 NBA draft, Jordan Mickey, to a four-year, $5 million contract, league sources told RealGM.

Mickey’s deal contains two years guaranteed, sources said. The Celtics announced the signing on Monday. It marks the richest second-round deal in league history.

During the NBA Summer League in Las Vegas and Utah, Mickey averaged 12.3 points, 7.9 rebounds and 2.4 blocks in eight games — separating himself as one of Boston’s best young assets. The 6-foot-8 forward also shot 52.7 percent from the field while playing 28.1 minutes a game.

With the Celtcs, Mickey will earn the contract of a first-round selection.

Mickey declared for the NBA draft after his sophomore season at LSU, where he averaged 15.4 points and 9.9 rebounds per game.