I don't think you can judge "Posey" vs. "big man" in a vacuum. Sure, having a quality big man off the bench would be tremendous, and potentially, a bench big guy could be worth more than Posey.
Who is that guy, though? Diop? I'm not sold. Patrick O'Bryant? Kwame Brown? There's just not a lot out there. Now, if Jeff Foster opted out, maybe you consider the MLE for him. Otherwise, I just don't see a lot of quality guys available.
I'm hoping that Mourning comes here. He'd be pretty much the idea fit, and if he signs, I can see it being for the LLE or less.