If Brown is out at all, or if he plays but is hampered by the injury, that's a big detriment to Boston—their biggest challenge has been offense, and Brown's one of their best scorers (as well as one of their best defenders).
Homecourt will be crucial, of course, but even with that advantage I think the Cs lose this series, even if Brown is 100% the entire series. They barely beat a team with a Giannis-Middleton combo, and I think the Embiid-Simmons combo is even better.
Then again, with how this Celtics team has come through so often this season despite long odds, it wouldn't surprise me if they won the series.