By Baxter Holmes, Globe Staff

Celtics assistant Mike Longabardi, who was Doc Rivers' "defensive coordinator" in Boston, has accepted an offer to become an assistant for Phoenix Suns head coach Jeff Hornacek, a league source told the Globe.

Longabardi becomes the first assistant coach to leave the Celtics this offseason, which has been filled with questions about whether Rivers will return to coach the team.

Aside from assistant Jay Larranaga, whose contract has one season remaining on it, all the other Celtics assistants' contracts are set to expire at the end of this month.

Longabardi joined the Celtics' staff prior to the start of the 2007-08 season.