Veteran NBA assistant coach Sam Cassell agreed to join the Boston Celtics' coaching staff under Joe Mazzulla, sources told Shams Charania of The Athletic.

Cassell was a lead assistant for the Philadelphia 76ers for the past three seasons under Doc Rivers, who was fired following the team's second-round exit.

A former NBA guard, Cassell spent the latter half of his final campaign as a player with the Celtics, winning the 2008 championship.

Three Celtics assistant coaches - Ben Sullivan, Aaron Miles, and Mike Moser - recently left to join former Boston bench boss Ime Udoka with the Houston Rockets, Gary Washburn of the Boston Globe reported last week.

Cassell brings experience to the Celtics' bench. He's been an assistant coach for 14 seasons, spending time with the Washington Wizards and Los Angeles Clippers before joining the 76ers.