The Wizards' trajectory the past three seasons - two losing years and then a playoff appearance - can be explained most simply by three factors. One would be that John Wall was hurt the first two years, then they got Russell Westbrook. Another would be that they had historically bad defenses the first two years, then improved substantially on that end last season.
The third would be rebounding and it's directly related to the other two reasons. They were dreadfully bad at rebounding in 2018-19 and 2019-20, then they got good at it and made the playoffs.
That brings the focus on Westbrook and his departure. Now that he's gone, will the Wizards struggle to rebound again? Robin Lopez moving on could also hurt, as he is one of the best players in the NBA at boxing out. He and Westbrook made a great one-two punch; Lopez muscling big men out of the way so Westbrook could swoop in and clean the glass.
Westbrook, in particular, will be missed. He averaged 11.5 rebounds per game last season, ranking fifth in the NBA. He brought in more than twice as many rebounds as the next qualified player on the Wizards' 2020-21 roster, Daniel Gafford (5.6). Repeat: more than twice as many.
Westbrook helped the Wizards improve their rebounding dramatically. Just look at where they ranked in key rebounding categories the past three seasons, 2020-21 being the one Westbrook played for them:
2020-21: 8th in rpg (45.2), 6th in drpg (35.5), 15th in orpg (9.7), 19th in reb% (49.4), 16th in dreb% (73.6), 24th on oreb% (25.0)
2019-20: 28th in rpg (42.0), 28th in drpg (31.9), 17th in orpg (10.2), 30th in reb% (47.8), 27th in dreb% (71.2), 19th in oreb% (26.2)
2018-19: 27th in rpg (42.4), 27th in drpg (32.7), 23rd in orpg (9.7), 29th in reb% (47.7), 28th in dreb% (70.6), 25th in oreb% (25.2)
Rebound percentage is a category worth double-clicking on here. From April 7 to the end of the 2020-21 regular season, when the Wizards went 17-6, they were ninth in rebound percentage (51.5).
It can also be a good indicator of bad teams. Among the bottom-10 NBA teams in rebound percentage last year, only one (the Heat) made the playoffs. Six of the seven worst teams in the NBA based on record were in the bottom-10 of rebound percentage.
The Wizards know very well how important rebounding percentage is. Their assistant coach, Dean Oliver, invented and popularized the Four Factors theory, which identifies the most important criteria which determine wins and losses in the NBA. Per his equation, rebounding is the third-most important factor, behind shooting and turnovers, and accounts for 20% of the sum.
Now consider the Wizards' record the past two seasons when they outrebound their opponents, compared to when they are outdone on the glass.
2020-21: 20-11 when they won rebounding category, 12-24 when they were outrebounded
2019-20: 10-7 when they won rebounding category, 14-36 when they were outrebounded
(each year they tied opponents in rebounds five times)
In short, rebounding matters quite a bit. That is especially true on defense, where a defensive stop is not completed after a missed shot until the rebound is secured.
The Wizards' current roster, revamped by a series of offseason trades, features no one who averaged more than 6.2 rebounds per game last season. They also have zero players who ranked in the top-50 of the NBA in rebounds per 36 minutes. Westbrook is no question going to be a noticeable loss in that regard.
Now, all of that said, boxing out is unlikely to be a problem, even with the exit of Lopez who led the NBA in box outs per-36 minutes last season and was fourth in total box outs. The Wizards still have Gafford, who was ninth in box outs per-36 and newcomer Montrezl Harrell is perennially one of the best at the trade.
Harrell was 11th in the NBA in total box outs last season, third the year before and ninth in 2018-19. He's a specialist at the dirty work and, like Lopez, is completely fine doing the little things to help out his teammates.
Harrell also did a solid job of grabbing his own rebounds last season, pulling in 9.8 per-36 minutes which should help offset the loss of Alex Len (9.8 per-36). And Thomas Bryant's return should play a factor, as he's averaged 10.4 rebounds per-36 minutes for his career; not elite for his position but solid.
There may also be some upside for the guys who came over from the Lakers (Harrell, Kyle Kuzma and Kentavious Caldwell-Pope) to tap into, as they had to fight for rebounds with guys like Andre Drummond, Anthony Davis and LeBron James in L.A. Kuzma, for instance, averaged a career-high in rebounds (6.3) as a rookie in 2017-18 before any of those three arrived. The same goes for Caldwell-Pope (5.2 rpg in 2017-18), while Harrell's career-high (7.1 rpg) came the year before he signed with the Lakers in 2019-20.
The Wizards have the potential to be a decent rebounding team if everyone does their part. They have guys who can box out and a few who are adept at cleaning the glass.
But the reality is they have a big hole to fill with Westbrook gone and they know as well as any team just how important it will be to do so.