To be honest, of the contenders he could go to, I think it is least damaging if he goes to the Cavs. I don't think he is a great fit next to Lebron defensively. I also think they were already an incredibly tough, and feisty team, so Posey's strengths don't stand out as much.
Don't get me wrong, he definitely helps the Cavs, but I was much more worried about him going to a team like LA, Toronto, Portland, or some other team that he could really fit in well with.