I think there's some overreacting to the Bucks game here. It was the worst matchup possible for the team maybe. How the Celtics are now without Gordon, the Bucks actually are a pretty similar team trying to do similar things (be long, versatile, and get in transition). Without running crisp sets (or actually plays correctly at all, if you heard about Stevens threatening to sit guys because of how badly plays were being run) you will not counter their length.
I think the Bucks are a bad matchup. Before the season began, I actually had the Bucks beating the Celts (with Hayward) in the playoffs.
That said, the Celts are a very young team. In many respects, they are now a lot like last year's team, except that they are simultaneously bigger and more talented on the wing, but also much less experienced.
We should not expect the team to be crisp and consistent for a while. Late game execution will be problematic. It'll probably come down to Kyrie hitting difficult outside jumpers.
I believe in Brad's ability to get these guys working together and to help Kyrie figure out how to be a true superstar -- i.e. a guy who doesn't just get his, but who makes his whole team function more effectively. I just think it's going to take time.
If this team is around .500 about halfway through the season, that should be counted as a victory. They can try to go on a run in March and April to end up closer to 50 wins than 40 wins.