Open layups are clearly a problem, but Rosco didn't say they weren't. He said that "one of the most damaging plays is ... the offensive rebound," and he's right.
I don't necessarily disagree, but I guess my point is that simply improving the defensive rebounding wouldn't really move the needle that much for this team unless you're adding a guy who can also help you in the pick and roll and near the rim without sacrificing the integrity of the perimeter defense.
Defense is about the effectiveness of a five man unit, not what one guy does. That is more true now than it was 10 years ago, because isolation, strong-side basketball has become less common.
Perhaps the Celts could have added a player like that by going after Noel or Cousins, but a move of that kind would have had long term implications.
Giving up a couple seconds or a future 1st for a player who maybe helps on interior but isn't part of the puzzle long term may have made some difference for this season, but still wouldn't have given the Celts the pieces they need to have a realistic shot against the Cavs. So in that case, while you're clearly sacrificing the chance to play better basketball this year, what are you really gaining by giving up assets, even minor assets, for the short term improvement of this year's team?
Rebounding may not be such a glaring issue as soon as next season. Surely Ainge is thinking he's going to try to add somebody like Blake Griffin, or perhaps try to trade for a guy like Deandre Jordan (if Griffin and Paul sign with other teams). Plus, you have Ante Zizic likely coming over. He will be a rookie, but I expect he'll help on the boards right away. Perhaps Mickey will finally be ready to give the team some minutes as well.
In that context, giving up a 1st or a couple 2nds this season to trade for, let's say, Kyle O'Quinn, might seem like a major overpay if you end up losing in the 2nd round anyway and then the guy you traded for becomes redundant after you make a couple major moves in the off-season.
Basically, I think rebounding is a significant short term issue for the team, but there wasn't an obvious solution available to address that problem, at least not one that would be worth the cost. If the team can get a guy like Bogut or Terrence Jones on the waiver wire, why give up a pick at all?