If any game is not a must win game, the expectations are to be a mediocre team that is OK with not being a national champion competitor.
I find this talk about must-win as being kind of silly. Maybe that applies to very-long season sports like basketball or baseball with lots of games - having an off night is no big deal. But especially in football (or anything you want to be 'great at'), every game is must win, no loss is acceptable or good.
If you need to tell yourself it isn't a must win to 'feel better' or manage expectations, alright - fine. We all get by with being a fan somehow. But in any sort of serious way, this must-win stuff is ridiculous.
You're not going to get handed a trophy just for thinking about how great it would be to win a football game - you have to earn it. You 'must. win.' it.