I think the “must win” comes from the fact there are certain games that are thought to have a higher probability of being won. For instance, division games against the Jets and Dolphins are must wins because you might not be able to beat NE at all. The must wins are those they can’t afford to lose because it means depending on beating tougher teams down the road, which the Bills haven’t shown any ability to do so far this season, other than maybe Tennessee. The Bills “must win” the gimme games against bottom feeders because they may not be equipped to beat the top teams yet.