Boston Celtics guard Kemba Walker received a stem cell injection in his left knee and won't return until the first week of January at the earliest.

The Celtics said Tuesday that Walker received the injection after visits with multiple specialists in early October. He was put on a 12-week strengthening program.

The team said Walker is expected to resume on-court activities in early December. In early January, the team plans to update his availability for games.

Walker, 30, is entering the second year of a four-year deal with the Celtics. He averaged 19.6 points in the playoffs, and team president Danny Ainge said it was clear Walker was not himself due to the knee injury.

While Walker wouldn't point the finger at his three-time surgically repaired left knee, Ainge and head coach Brad Stevens said it was hampering him throughout the 2 1/2 months the team spent at the ESPN Wide World of Sports Complex near Orlando after the season restarted this summer.

"I could see, even when he was (in Boston) before the bubble started, which is why he was shut down a little bit and doing strength training and trying to prepare himself for the playoff run and the intensity of the playoff run, but he was definitely not himself," Ainge said during a season wrap-up conference call.

"In fairness to Kemba, he doesn't want to say that. He doesn't say that to our coaches. He doesn't say that to you, the media. He doesn't say that to me. I haven't heard one excuse from him. But watching the games, even the games we won, even the games where he played well, I could tell he wasn't the same physically as he was in October, November, December. So we're going to try to get that Kemba back."

Walker averaged 20.4 points, 4.8 assists and 3.9 rebounds last season. The four-time All-Star played the previous eight seasons for the Charlotte franchise.

Walker first mentioned knee soreness to the Celtics before the All-Star Game in February. At that point, he had the knee drained of fluid and received an injection of Synvisc.