Boston Celtics guard Kemba Walker received a stem cell injection in his left knee and is out until at least the first week of January, the team said Tuesday.

Walker received the injection after consultation with multiple specialists in early October and was put on a 12-week strengthening program. The Celtics said he is expected to return to on-court activities in early December, and an update on his availability to play in games will be provided in the first week of January.

Walker, 30, averaged 20.4 points, 4.8 assists and 3.9 rebounds last season, his first with the Celtics after signing a four-year contract in free agency.

In the playoffs he averaged 19.6 points, 5. 1 assists and 4.1 rebounds as he helped the Celtics advance to the Eastern Conference finals, where Boston fell to the Miami Heat in six games.

The Celtics also announced Tuesday that newly signed forward Tristan Thompson suffered a minor hamstring injury during an offseason workout before he signed with the Celtics and will be limited during the first week of training camp.

In addition, guard Romeo Langford’s rehab has been proceeding as planned after he underwent surgery to repair a torn ligament in his wrist on Sept. 22. His expected return from the date of the procedure was four to five months, the team said.

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