Celtics’ Robert Williams receives platelet-rich plasma injection, but return timeline not changed

Robert Williams often appeared hobbled during the playoffs last season and later acknowledged that he was playing through pain.Jim Davis/Globe Staff

Celtics center Robert Williams received a platelet-rich plasma injection in his left knee on Monday, but the procedure is not expected to affect his previously announced 2-3 month timeline for a return, according to an NBA source.

Williams tore his meniscus last March and missed about a month before returning during the first round of the playoffs, but he often appeared hobbled during the postseason and later acknowledged that he was playing through pain.

Last month, Williams underwent a maintenance procedure on the knee. The Celtics initially believed he would resume basketball activities in 4-6 weeks but later extended the timeline to 8-12 weeks. But Monday’s injection is not expected to affect that.

“Just some recurring problems,” Williams said last month. “Obviously, it took a toll on me mentally. I wanted to be there but, man, all I could focus on is rehab now, man, just getting back on the court.”

No deal

The Celtics and forward Grant Williams were unable to agree on a contract extension prior to Monday’s deadline.

Williams will now become a restricted free agent next summer. A league source said that despite the fact that the sides could not agree on a deal, Williams remains an important part of the Celtics’ future. When he is a restricted free agent, the Celtics will be able to match any offer sheet that Williams signs with another team.

Williams celebrates after a fourth-quarter bucket during Tuesday's win over the Sixers.Barry Chin/Globe Staff

Last season, Williams, 23, averaged 7.8 points and 3.6 rebounds while shooting 41.1 percent on 3-pointers. He is expected to assume a larger role this season with forward Danilo Gallinari out for the year after tearing his ACL, and Robert Williams sidelined.

Mazzulla’s debut

The Celtics’ Joe Mazzulla made his head coaching debut in Tuesday night’s opener against the 76ers. Mazzulla, 34, was named interim coach in place of Ime Udoka, who has been suspended for a year because of violations of organizational policies.

Mazzulla, a Rhode Island native, mostly shrugged off the significance of his big night.

“It’s a great opportunity, but it’s more important that we focus on players, the team, and what we’re trying to do this year,” he said. “And so, yes, there’s small pockets of [what it means to me], but it’s more about the guys, and it’s more about what we’re trying to do.”