This is in the category of much ado about nothing. Brown shouldn't have said it but really all he said was that the Celtics don't have Robert Williams and that impacts their interior defense. I don't think this is going offend Horford or Theis and make them play better or anything like that. It does perhaps tip the Nets hand some on how they plan to attack the Celtics but I suspect that Celtics would have realized this on their own.
It is a bit of a distraction for the Nets so happy to see that. I am sure Horford and Theis will be asked about it along the way and I expect the answer to be "yeah, Rob Williams is a great player but next man up".
This by itself is not big deal in my opinion but it is the kind of thing the Nets may continue doing. Maybe Kyrie says something stupid next. Maybe some bickering starts in the locker room. Durant will need to keep all this together for team. I think he will be able to.