The Celtics had as much momentum as any team in basketball, winning 19 games of 22 games behind the league’s best defense and an offense in rhythm. Then Robert Williams’ knee injury happened.

Boston has since lost two in a row as they adjust to life without Williams as their starting center. Good news? He’s reportedly on track to return within 4-6 weeks, which would get him back in the lineup in time for the start of the second round if and when the Celtics advance.

Bad news? They still need to replace his 10 points and 9.6 rebounds per game, and elite shot-blocking, as they hit the stretch run entering the playoffs. Williams’ absence will have a significant effect on both the Celtics’ in-game lineup choices and the potential matchups they face in the first round.

Here’s a look at how the loss of Williams will affect the Celtics for the next month and a half…

Increased Roles

Dec 27, 2021; Minneapolis, Minnesota, USA; Boston Celtics forward Al Horford (42) talks to his teammates during a timeout against the Minnesota Timberwolves at Target Center. Mandatory Credit: Nick Wosika-USA TODAY Sports

In Williams’ absence, bigs like Al Horford and Daniel Theis will need to step up amid their increased minutes, both defensively and to find the occasional opening to score around the rim.

For Horford, his presence and scoring contributions have been similar to Williams, as both average around 10 points per game. Horford will have to return to his output from his first stint with the Celtics, and help limit the opposing bigs down the stretch. A potentially huge meeting with Giannis Antetokounmpo and the Milwaukee Bucks still remains on the schedule. Horford’s ability to both chip in offensively and engage with opposing post players will be his best chance to replicate Williams’ skill set.

Bringing back Theis at the trade deadline now weighs even larger, with the depth he provides down low. Since rejoining the Celtics, Theis has added 4.5 rebounds per game to support the team on the boards. Theis won’t be nearly as productive as Williams at either end, but he will have a hand in occasionally spelling Horford on defense.

Horford and Theis will need to step up regardless of matchup, but there are a few notable teams in the Eastern Conference to know about while Williams is out.

Brooklyn Nets

Mar 6, 2022; Boston, Massachusetts, USA; Boston Celtics forward Jayson Tatum (0) and Brooklyn Nets forward Kevin Durant (7) react during the first half at TD Garden. Mandatory Credit: Paul Rutherford-USA TODAY Sports

The Boston-Brooklyn matchup has led multiple highlight moments of the Celtics’ season, from leading 28-2 in the first quarter at Barclays Center to Jayson Tatum’s 54-point performance in early March. Nonetheless, an offense led by Kevin Durant and Kyrie Irving can present problems for every team in the Eastern Conference.

Brooklyn’s addition of Andre Drummond, who averages 12.5 points and just under 10 rebounds per game, provides a threat in the post for the Celtics to defend. Internally, Boston will need to make up for Williams’ production against Brooklyn this season, averaging a double-double against the Nets this season along with eight blocks.

Toronto Raptors

Mar 28, 2022; Toronto, Ontario, CAN; Toronto Raptors forward Pascal Siakam (43) grabs a rebound from Boston Celtics center Daniel Theis (27) and guard Derrick White (9) during the overtime at Scotiabank Arena. Mandatory Credit: John E. Sokolowski-USA TODAY Sports

The Celtics are 2-2 against the team north of the border this season. Monday’s loss to the Raptors represents everything that the Celtics cannot allow their opponents to do in a potential first-round series. The Raptors are well-coached and offer scoring in the backcourt with Fred VanFleet.

The biggest factor for Toronto, however, is the impact of Pascal Siakam. As the team’s leading scorer with 22.5 points per game, the veteran forward feasted against a depleted Celtics frontcourt with 40 points and 13 points in Monday’s game. Boston must find a way to limit Siakam or he could take over a series without Williams on the floor.

Cleveland Cavaliers

Dec 22, 2021; Boston, Massachusetts, USA; Boston Celtics guard Jaylen Brown (7) shoots against Cleveland Cavaliers guard Denzel Valentine (45) in the second half at TD Garden. Mandatory Credit: David Butler II-USA TODAY Sports

The Cavaliers offer potential in the team’s first winning season without LeBron James since 1998. Boston is 2-1 against Cleveland this season, but the Cavaliers have shown that they can play with momentum and serve as a dark horse in the playoff love.

Even as an older veteran, players such as Kevin Love, averaging 13.5 points per game, can create a productive frontcourt for Boston to defend. The Celtics will once again need to fill in for Williams’ contributions against Cleveland. In two full games against the Cavaliers this season, Williams posted two doubles, one for 12 points and 16 rebounds followed by 21 points and 11 rebounds.