Believe it or not Warrick is equivalent to Frye in the number of rebounds they collect individually but I suspect Frye's blocking out helps team rebounding a fair amount. But on defense Hakim is not only undersized he's usually mentally out of it - the more minutes he plays the more other teams will figure out he can be exploited. Lopez and Frye did fairly well covering for each other but Robin is going to be in a world of hurt depending Hakim to have his back, not mention the fact that Hakim will switch onto a smaller player whenever the opportunity presents itself and that he way over commits chasing guards defending the pick and roll. Robin better hope they start Gortat, who, at least, has had a good amount of experience playing alongside Warrick.
I don't expect Gentry to give Siler a shot but in my book Warrick is bad enough that there's almost no downside to trying Garret... heck our rebounding may well improve a bit.
