At times he is absolutely the worst defender in the league, but that's not really the main issue. He simply isn't as good of a shooter as we thought. He is currently sitting at 38%, but many of them are good open looks. He needs to be shooting in the low 40% if he is going to establish himself as a useful player, especially if he is going to play passive defense. At this point, he is on par with a washed up Klay Thompson.
I was also pretty pissed at Tari last night. He gave up a bunch of lay ups on one-on-one drives. He is playing for an extension and based on his injury record and inconsistency, I'd look to trade him while his value is high. His 3pt percentage is going to come down as the season progresses (and he is probably due for another injury). If we can get a real difference maker, I'd be happy to let Reed go with him.