This is most decidedly *not* a must win. There are any number of scenarios that have us winning the division while losing this game. For Pete's sake, we can lose this game and still finish two games up on the other NFCE contenders.
However, to your point, losing this game or the game in Arizona would put a lot of pressure on us to beat the Eagles, which is something I would rather not count on.
Split with the Giants? Yeah, I'm OK with assuming that. Beat TB? Yeah, that too. Beat the Eagles with the division and playoffs on the line? We've done it before, but I'd be really nervous.
If the Cowboys put forth their best effort, I think they win easily.
If they put forth their best effort and lose, so be it. If that happens, no one should fold up the tents and declare the season over, though. The Cowboys won't do it, and their fans shouldn't either.