I'm excited on the offensive end of the floor for an Irving-Brown-Hayward-Morris-Horford lineup.
That's gotta be the worst rebounding team in the league though.
True, but it doesn't get better as you go down the roster.
People tend to lump offensive and defensive rebounding together, but that doesn't give a clear picture.
Offensive rebounding is part of offense, and it's a function not just of how beasty your bigs are but of how you play offense; San Antonio in this era, for example, has not been a good offensive rebounding team, while its offense has been consistently elite, innovative, and widely imitated. You could make a good case that offensive rebounding is among many choices for having an efficient offense, and by no means the most desirable of them.
Defensive rebounding, on the other hand, is not optional. If the rest of your defense is good, and your defensive rebounding is bad (see, for instance, the 2016-17 Celtics), then your whole defensive efficiency suffers. Boston had outstanding FG defense last season (fifth in the league), while the defensive rebounding dragged them down (27th in defensive rebounding, 13th in total defense).
Unfortunately, the prospect for the upcoming season is
worse. Olynyk was the top defensive rebounder last season, at 20.7; replacing him with Baynes (21.6) is likely a marginal upgrade. But after that it tails off steeply. If Morris starts, we might expect an increase from his minuscule 12.8 - he'll be called on to do that; but is it likely that he'll be better than Crowder or Johnson? Morris would have to have a career year plus on the defensive boards.
No need to belabor the point - this is a big deal and something to keep an eye on from game one.