Sources: Celtics adding Sam Cassell to Joe Mazzulla's staff

Brad Stevens stands by Celtics coach Joe Mazzulla (2:26)

Brad Stevens is confident in Joe Mazzulla and his future as the Celtics coach. (2:26)

The Boston Celtics are hiring Sam Cassell as an assistant coach on Joe Mazzulla's staff, sources told ESPN's Adrian Wojnarowski on Sunday.

Celtics president Brad Stevens announced last week, after the team lost to the Miami Heat in seven games in the Eastern Conference finals, that Mazzulla would return as coach but said he wanted to hire a veteran assistant to his staff.

Cassell, 53, will fill the spot vacated when Damon Stoudamire left to become Georgia Tech's coach in March. Three other Celtics assistant coaches -- Ben Sullivan, Aaron Miles and Mike Moser -- are reportedly leaving to rejoin former Celtics coach Ime Udoka, who was named head coach of the Houston Rockets in April.

Mazzulla, 34, led the Celtics to a 57-25 record and the No. 2 seed in the Eastern Conference in his first season on the bench. He took over when Udoka was suspended for the season in September for multiple violations of team rules. He reportedly had an intimate relationship with an unidentified female staff member.

Cassell was an assistant on Doc Rivers' Philadelphia 76ers' staff the past three seasons and was on Rivers' staff with the LA Clippers before that (2014-15 through 2019-20). Rivers was fired by the 76ers last month. Cassell began his coaching career with the Washington Wizards and spent five seasons with the franchise (2009-10 through 2013-14).

Before becoming an assistant, Cassell played 15 seasons in the NBA and was part of three championship teams, including the Celtics in 2008. He played for eight teams during his career, averaging 15.7 points and 6.0 assists in 993 regular-season games.