The Boston Celtics lost Gordon Hayward in free agency, and now they won't have their All-Star point guard to begin the 2020-21 season. Kemba Walker received a stem cell injection in his left knee that will keep him out at least through the first week of January, the team announced Tuesday morning. 

Per the team injury report, Walker consulted with several specialists in early October, and it was decided that he would received the injection in his knee, and be put on a 12-week strengthening program that would take him through the remainder of the year and into January. 

Walker is expected to return to on-court activities in early December and a further update on his status will be given in January. 

Walker first started to have issues in his left knee around All-Star Weekend in February, which limited him to just four games after the break before the season was postponed due to the COVID-19 pandemic. Before the Celtics traveled to Orlando for the NBA restart, Walker was dealing with discomfort, which led them to put him on a minutes restriction during the team's seeding games. 

By the time the playoffs rolled around, though, Walker was no longer on a minutes restriction and averaged 19.6 points, 5.1 assists and 4.1 rebounds in 17 games for Boston. However, in a recent appearance on the "Bob Ryan and Jeff Goodman NBA Podcast," head coach Brad Stevens said Walker was still not 100 percent healthy, and it would be a while before he's going "full speed."

When asked about Walker's knee during media availability on Tuesday, Celtics general manger Danny Ainge said this upcoming season will "tell us more" in regards to Walker's health moving forward. While Ainge feels good with the plan they've put Walker on, he thinks that perhaps they brought him back too soon in the bubble. 

While not having Walker at the start of the season is a blow for Boston, especially since the Celtics lost some scoring depth after Hayward was traded to the Charlotte Hornets, having him at full health is paramount for them as they contend for a title. In a shortened season, it will be important for Boston to manage Walker's minutes wisely so he's not overloaded and doesn't risk re-injury to his knee after he's finished with this 12-week strengthening program.