Effort is the story for Celtics on D and rebounding, to be sure. That's awful.
Offensively, this is more about shooting and officiating than process. There's some bad choices---in particular, from end of first to mid-second they had a sloppy stretch. But they've created a fair number of good looks both in paint and for three and cashed very few. Outside that's about shooting variance; in paint it's a mix of that and getting bodied.
I expect they'll come out better around all of that in second half. But it's a big hole to dig out of now.