Sullinger had moved into the Celtics' starting lineup recently, and had become a pivotal part of the team's nucleus.
After experiencing pain in the past two weeks, Sullinger had to leave Wednesday night's victory over Sacramento in the first quarter and missed the Celtics' practice on Thursday after the back pain worsened.
The loss of Sullinger is the second significant blow to the Celtics this week, who lost point guard Rajon Rondo for the season with a knee injury on Sunday.
Sullinger, an All-American out of Ohio State, dropped out of the lottery and landed with the Celtics with the 21st pick in the June NBA draft after predraft medical examinations raised questions about the long-term durability of his back.
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