Boston Celtics center Robert Williams III. Winslow Townson-USA TODAY Sports

The Boston Celtics’ Eastern Conference title defense is already hitting a pothole.

ESPN’s Adrian Wojnarowski reported Tuesday that Celtics big man Robert Williams will be undergoing an arthroscopic procedure on his left knee and is expected to miss four-to-six weeks. Wojnarowski adds that Williams was plagued by the knee injury in the playoffs after having meniscus surgery in March but played through it.

Sean Deveney of Heavy.com adds that Williams will likely now be sidelined for all of training camp and into the early weeks of the regular season.

The 24-year-old Williams is perhaps the Celtics’ most energetic young player. He served as Boston’s starting center during their Finals run last season, blossoming into a nightly double-double candidate with top-shelf rim protection plus the ability to switch out onto the perimeter and contest jumpers as well.

But Williams was badly hampered by his knee issue throughout the playoffs after returning to action less than a month after his meniscus surgery. One report even detailed just how much Williams went through to play.

Training camp begins later in September, and the regular season starts for the Celtics in exactly four weeks on Oct. 18. That means Boston may have to rely on 36-year-old Al Horford and backup bigs like Luke Kornet and Noah Vonleh to sop up the 5 minutes that Williams will vacate in the early going.

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