Powe hasn't been good in this series. He's had one good game, one average game and all the rest are below par for him.
This is what happens to players like Powe, he's completely dependant on his teammates to be a factor offensively and his teammates aren't living up to their end of the bargain. Not him, his teammates. That's why he hasn't been as big of a factor as normally.
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I don't think Zaza has been a problem for him at all. He's done fine on Zaza. It's the quicker more atlethic bigs like Horford (and they have to hold Smith away from him like the plague) that have gotten to him. Not a great series/matchup for Powe.
I would only bench Powe if BBD got the task of defending Al Horford man-to-man, I wouldn't play BBD ahead of him in this series for any other reason. Powe simply offers more than any other Celtics backup big man in this series.
This team needs his movement off the ball, his cuts to the rim, his screen and rolls desperately. They also need his offensive rebounding and finishing desperately. Everytime Josh Smith goes for a block there's an offensive rebound and easy dunk available and Leon Powe is single most likely player, by a huge margin, to get that board and putback. That's why he should play.