Maybe I'm overthinking it -- but Brissett working out seems really important to me. At each other position we have some more coverage for likely injuries:
Point Guard -- White, Brogdon, Pritchard can all cover. Banton if we really need it.
Shooting Guard -- Brown, White, Brogdon, Pritchard, Hauser, Svi. We have people who have played NBA minutes
Small Forward -- Tatum, Brown, Hauser, Svi, Walsh, Brissett
Bigs (using collective because Porzingis and Horford could do either) -- Williams, Porzingis, Horford, Brissett
Maybe it's because I don't really consider Kornet to be a viable player and Banton + Walsh are only borderline. So at other positions I think if things go sideways, you have some cover from people who have proven they can play NBA minutes at a reasonable level. You can down select the rotation and get to 48 minutes. If Horford ages and you lose Porzingis or Williams for a while, Brissett is playing a big-time role. There's no way around that. Plus he's insurance at the 3.
I'm actually more bullish on him than others are, I think -- but he's going to have to step up even more to get to the level the Celtics are going to need him to play. He's almost certainly going to play almost 20 minutes a game with nobody behind him to cover. Nobody else below the obvious top 7 is in that position.