Boston Celtics center Robert Williams III was ruled out for Monday's Game 4 matchup with the Milwaukee Bucks due to left knee soreness, the team announced.

Head coach Ime Udoka told reporters Williams had a "flareup" of the soreness on Monday.

Grant Williams moved into the starting lineup to replace Williams.

It's a tough break for a Celtics team that already finds itself down 2-1 in the series despite Khris Middleton missing the first three games as he recovers from an MCL sprain.

The Williams loss is a big one. The 24-year-old anchored Boston's impressive defensive revival this season, averaging 9.6 boards and 2.2 blocks per game to go along with 10.0 points per contest in 61 starts.

Williams' offensive impact comes mostly from lobs and putbacks, allowing him to shoot an impressive 73.6 percent from the field this season. But his real footprint comes on the defensive end.

With Williams on the court this postseason, the Celtics have given up just 98.1 points per 100 possessions, per NBA.com. That jumps up to a 108.8 defensive rating when he sits.

The Celtics will still have Al Horford and Daniel Theis available at the 5, both capable options. But losing Williams for Game 4 means Boston will be without its best rim protector and one of the big men it can throw at Giannis Antetokounmpo.

It's also something of a long-term concern this postseason, should the Celtics survive the Bucks. Williams suffered a torn left meniscus in March and missed around three weeks, returning for Game 3 in the team's first-round sweep of the Brooklyn Nets.

Granted, a loss in Game 4 would leave the Celtics' season on life support. But if Boston can fight back into this series, the status of Williams' knee going forward will be one of the primary storylines to follow.