The FT's thing is a worry. It's the best way to get easy points and the Celtics, right now, don't have anyone who consistently gets to the line a la Pierce or IT.
Kyrie's game is about avoiding the defender, not taking it at them so his FT's have always been low but he's even lower now.
Horford has never been that kind of player either, he's just not a banger. Hayward isn't right yet so he's not getting to the line. As someone else said, Brown should be getting to the line a ton but he still struggles reading the defense and bending it to where he can put himself in position to get contact.
Tatum is the best bet. For his uptick in shot attempts he should be getting to the line more. He needs to dust off that Paul Pierce video and work on his pump fake.