My broad thoughts so far...
I think it can't be forgotten that Boston still is a team that very much lacks talent. They overachieved last year thanks to Stevens, effort and execution.
Maybe it's because I just watched a marathon of 5 "Rocky" movies and the new movie "Creed" (which is phenomenal), all the elements of the Rocky movies are relevant to this team. When you are lacking talent, you need to prepare 110%, you need to practice 110%, you need to execute at 110%, and you need to give it 110% effort. If you don't do these things, you don't stand a chance.
The problem with that is it's incredibly hard to sustain. If you don't give it your best effort (Apollo in Rocky 1), get a big head (Rocky in Rocky 3), underrate your opponent (Apollo in Rocky 4)... You'll fail.
Boston's going to slip into this all season. They'll win games when they go hard and execute... but then they'll start to think they can win games at 80% and get kicked in the face. We saw this happen after they faced a couple cream puff Euro teams this pre-season and paid the price at the start of the season... it mirrored Rocky's first performance against Clubber Lang after Micky fed him some cream puff opponents and Rocky didn't train hard. We saw this happen after we crushed Brooklyn, started thinking we were world-beaters and then proceeded to get crushed the next two games... which mirrored Apollo's lack of humility when he stood toe-to-toe in an untelevised third bout with Rocky (in which Rocky likely let him win) and then thought he could wipe the floor with Ivan Drago. Don't underrate your opponent or you might end up dead in the ring.
Relying on effort and execution is a fragile thing. Brad will kick some sense into them, they'll run up a snow mountain and show up correct in the next game... probably play a lot better... they'll win some games in spite of their talent, but if they slip again they'll get crushed. My main concern is that if players start getting injured, they lose a few games, players start getting frustrated, or they start tuning out Brad... the lack of talent can get ugly quick. I still think they'll win about 45, though.