O'Neal will have season-ending surgery on his left wrist, the team announced on Monday.
The 33-year-old big man, who was in the midst of his second season with the Celtics, hasn't played since Feb. 20 because of the injury. O'Neal suffers from a chronic degenerative wrist condition that was exacerbated after a fall during that contest.
"Jermaine worked hard to get himself in condition to play this season despite his ongoing wrist issues," Celtics president of basketball operations Danny Ainge said. "He played through pain the entire year and gave us all he could, but unfortunately after the fall against Dallas there simply wasn't anything else he could do. We appreciate his contributions to our team over the last two years."
O'Neal averaged 5 points and 5.4 rebounds per game through 25 contests (24 starts) this season. He averaged 5.4 points and 3.7 rebounds in 24 games last season, which was also plagued with injuries.
The news itself isn't all that shocking, but it's still a crucial blow to a Celtics team that lacks frontcourt depth.