Erman, who was fired by the Warriors earlier this month for a violation of company policy, worked as a Celtics' associate coach for parts of four seasons prior to joining Mark Jackson's coaching staff in Oakland.
"Darren worked with us for four years and he was one of the hardest working, most competent and intelligent members of our organization," Ainge told Yahoo. "We welcome him back."
Sources told ESPN's Chris Broussard that Erman's violation was secretly recording conversations between the team's coaches and players.
Erman, who was Jackson's second assistant, would record coaches' meetings, meetings between coaches and players and informal discussions among coaches that took place in the team's coaches room -- all without the participants' knowledge, sources told Broussard.
Erman is a graduate and former student coach at Emory University, and his resume includes stops at St. Anthony High School in New Jersey (2003-05) and Brandeis University (2005-07), near the Celtics' practice facility in Waltham, Mass.
ESPN's Chris Broussard and ESPNBoston.com's Chris Forsberg contributed to this report.