Indeed, KO is in a sophomore slump.
He's become a focal point for opposing teams' defenses, especially in the first quarter. The word is out that if you can take out Kelly, you can effectively limit the Celtics offense.
Bother KO by getting physical and he gets frustrated just like good ol' Randy Moss. Miss a few shots + turnovers he gives up looking for his shot. Sure he'll focus on rebounding and making hustle plays, but when AB and Jeff Green take 2x as many shots per game, good luck.
Add some early fouls into the mix with questionable Stevens substitution patterns (yes lets sit KO and Rondo for most of the 4th quarter again?) and KO's output goes ka-put.
There's also a chemistry issue. Kelly looks positively out of sync with Sully and Jeff Green, especially in the fourth, since neither of them trust him with the rock. Bass has played more cohesively with the other starters but with that unit we've been dropping games....meh
On the positive side, anyone noticed how KO's defense has gotten better? He's bothering shots and fouling less. His basketball IQ is good for a few steals per game which is Bird-like, for sure and he has a nose for the ball.