It would be a lot easier if it was one or two things that you could point to and say "fix that." But as usual, it isn't that easy.
This season, offenses are having an easier time generating open looks. When you watch the film, the incriminating evidence isn’t glaring. This is still a comparatively efficient defense (11th overall) practicing those same principles, and the familiar cycle of movements is there, but point guards whom the Celtics used to send to remote outposts on the floor are finding their way to the middle. That old Celtics swarm doesn’t cause the same disruption it once did, which means offenses have more available options on the floor. Defensive systems take time to master, and it’s possible everyone will achieve the level of fluency necessary. The Celtics should hope so, because the team’s margin for improvement probably lies on that end of the floor.