It's not must win. It's actually the least important of the next three games. Win the two games after this one and a loss to the Pats isn't that big of a deal. Beat the Pats and they might be able to afford a hiccup against Houston. That KC game has almost become must win.
If the Bills can come out of this three game stretch 7-5, that would give them a conference record of 7-4 and a firm hold of a WC spot, particularly if one of those wins is vs. KC.