General manager Danny Ainge said Milicic asked for his release to deal with a personal matter.
"It's pretty much unemotional because we never really had a chance," Boston coach Doc Rivers said of losing him before the Celtics' game against San Antonio. "You know how I feel about stuff outside of basketball and how important it is."
Rivers also said the team was unsure what it was going to do with his roster spot.
Milicic played five minutes in one game this season. He had zero points, one rebound and two turnovers.