The Boston Celtics are finalizing a deal to hire Charles Lee as their lead assistant coach, sources told ESPN on Sunday.

The Celtics had long targeted the highly regarded Lee to join Joe Mazzulla's reshaped coaching staff, waiting out head-coaching searches by Detroit and Toronto to secure him. Lee was a finalist for both jobs, sources said, and was pursued for several top assistant coaching jobs in the offseason before landing with the Celtics.

Lee has been an assistant for former coach Mike Budenholzer for the past nine seasons -- including the past four with the Bucks -- and ascended to Milwaukee's top assistant job with the departure of Darvin Ham to the Los Angeles Lakers last year.

Lee will join former Philadelphia 76ers assistant Sam Cassell on the Celtics' coaching staff.

The Celtics reached Game 7 of the Eastern Conference finals under Mazzulla before losing to the Miami Heat. Mazzulla had taken over as head coach from Ime Udoka shortly before the start of training camp and only now is getting the chance to surround himself with a veteran and accomplished assistant coaching staff.