Kelly's defensive limitations are well documented, they're the same as when he was a college player.
Kelly's problems arise when he needs to get off the floor quickly without a running start, and when he needs lateral quickness. Of which he doesn't have.
For example, I've never seen Kelly take a JUMP shot, or jump hook, he's always flat footed on all his shots.
What's bothering so many, is this habit he has, the up-fake (that nobody is faked out by anymore), and then the turn down of the open shot, that he is very capable of making. Because being a stretch 4 is his NBA future. It show just how his confidence has evaporated. Kelly needs some new habits to develop for sure.
The coaching staff of the Celtics have to correct this, and fast.