Amir, Horford, and Jerebko all put up around 19-20% Defensive Rebounding Percentage last season.
That's not great, but it's just fine.
The fourth guy likely to be in the frontcourt rotation, KO, was around 16-17%, which is mediocre, but won't kill the Celts.
Not having a single rebounder who can really dominate the boards on his own does make it harder to go small without giving up boards. It helps to have wings and guards like Smart, Crowder, Rozier, and Thomas who are all competitive on the boards.