Right now it is safe to say Mike G is better than whoever his replacement would be because we don't know who that replacement is/would be yet. But it is entirely possible that the Bills end up replacing him with someone who is even better. It is not like Mike G is an elite running back. I have nothing against the guy. I think he is good and I like watching him play. But the Bills can do better. That is why I don't think this is a big deal at all.
I place higher value on offensive linemen than I do on running backs. So, there is that too. I think there are many good running backs to be had that could easily replace Mike G. I think they are there every year and in late rounds too. As long as the Bills identify that talent well, I think they will be just fine with whoever the number 2 back is this year. My biggest concern would be pass protection from a rookie missing on blitzes.