By Gary Washburn, Globe staff

Celtics assistant coach Lawrence Frank has agreed to become the head coach of the Detroit Pistons, according to a league source. Frank is the second of Doc Rivers's assistant coaches to depart for another team this week after Darren Erman left to be an assistant at Golden State.

This is also the second year in a row in which Rivers has lost his top assistant coach. Former Celtics assistant coach Tom Thibodeau became the head coach of the Chicago Bulls last season, guiding the Bulls to the Eastern Conference Finals before an eventual loss to the Miami Heat.

Both Thibodeau and Frank took the main controls of the Celtics' defense during their respective times in Boston. While the Celtics got older in the transition from Thibodeau to Frank, the team's defense still finished No. 1 in the league in opponent points allowed.

Frank has been an NBA head coach before, spending six-plus seasons as coach of the New Jersey Nets before an 0-16 start in 2009-10 cost him his job. The Pistons finished 30-52 last season, a far cry from the perennial Eastern Conference contender they were just a few seasons ago. Detroit selected Brandon Knight and Kyle Singler in this year's NBA Draft.