The Boston Celtics announced Tuesday that guard Kemba Walker received a stem-cell injection in his left knee in October and was placed on a 12-week strengthening program. 

The Celtics added that Walker will return to on-court activities in early December, and they will provide an update regarding his game availability in January's first week.

Since the 2020-21 season is set to begin Dec. 22, Walker will miss the start of the campaign.

Last season was his first in Boston after eight years with the Charlotte Hornets, and he averaged 20.4 points, 4.8 assists and 3.9 rebounds per game in 2019-20.

The four-time All-Star played an integral part in leading the Celtics to the Eastern Conference Finals after a disappointing 49-33 finish in 2018-19, a year that ended with Boston losing in the conference semifinals to the Milwaukee Bucks.

The C's excelled despite numerous injuries throughout their starting lineup. Of note, Walker missed time in 2019-20 because of left knee soreness in addition to a neck sprain and flu-like symptoms.

Marcus Smart replaced Walker in the lineup last season when the starting lineup was intact otherwise. The ex-Oklahoma State star averaged a career-high 12.9 points and 4.9 assists to go along with 3.8 rebounds and 1.7 steals per game.

Smart won't replace Walker's offensive production, but he's a hard worker on both ends who can fill up the stat sheet, and he made a career-best 2.3 three-pointers per game last season, which shows his offensive improvement.

Jayson Tatum and Jaylen Brown figure to carry larger scoring burdens in Walker's absence as well.