Robert Williams upgraded to questionable by the Celtics for Friday against the Magic

Robert Williams had a Sept. 23 maintenance surgery on his left knee, and is close to a return.Jim Davis/Globe Staff

Celtics center Robert Williams has been upgraded to questionable for Friday’s game against the Orlando Magic, and barring a setback he will likely make his season debut at TD Garden following Sept. 23 maintenance surgery on his left knee, according to a league source.

Williams has been taking part in scrimmages for several weeks, and he and the Celtics have said since last Friday that he is considered day-to-day. Williams and coach Joe Mazzulla hinted that the center could return against the Warriors last Saturday, but he ultimately missed that game and the two that followed in Los Angeles, primarily because he was working on his conditioning.

Last season Williams averaged 10 points, 9.6 rebounds, and 2.2 blocks and received second-team All-Defense honors. He tore the meniscus in his left knee last March and missed about a month before returning during Boston’s opening-round playoff series against the Brooklyn Nets. He missed part of the semifinals series against the Milwaukee Bucks due to lingering pain and appeared hobbled at times during the playoffs.

Robert Williams III, center, could be off the sidelines and back in action sooner rather than later.Barry Chin/Globe Staff

The Celtics were hopeful that he just needed an offseason of rest to fully recover, but when pain returned as he ramped up September workouts, the team decided surgery was the best option.

Forward Al Horford, who has missed the last five games due to being in COVID-19 health and safety protocols and then to be with his family for the birth of his daughter, is also listed as questionable for Friday.

Given the timeline and the opponent, it always seemed logical that Williams would make his debut during one of this weekend’s home games against the Magic.

It was surprising, then, when Williams and coach Joe Mazzulla both smiled and hinted that Williams might return against the Warriors last Saturday. He was ultimately held out of that game, as well as the two in Los Angeles that followed. Mazzulla cited Williams’s conditioning as the primary reason and insisted there had been no setbacks.

But Williams is not going to play big minutes when he returns anyway, and it would seem he and Mazzulla had a good sense of his conditioning and comfort levels 24 hours before tipoff against Golden State. And that was almost a week ago now. Maybe they were trying to affect the game plans of their opponents, but that’d seem like an unlikely reach. The whole situation felt a bit odd.