Things haven't been going well for Stevens since that disastrous game 7 against the Cavs at home, which should have never been lost.
Going overboard on the 3 has been a problem, when the strength of the team has always been its defense and running game--and ball distribution for easy shots. That includes the 3. But a lot of those have been wasted shots with no ball movement and nobody near the basket.
The other problem is the Hayward factor. He isn't ready to be a starter on this team right now, but Stevens is utilizing him in the starting rotation, to their detriment. He slows down the running game and weakens their defense. Irving hasn't been all that sharp at times, either, and Rozier maybe is right in sensing he should be playing more. But it's probably Hayward who is taking minutes away from Rozier, Brown and Tatum, who should be the future of this team.
The Celtics have a chemistry problem, and it's up to Stevens and Ainge to sort it out and do the right things which don't weaken this talented team.