I disagree that its must win for the division. Of course its a bit of a dagger if they are down 5 with 29 to go (I've seen people say 30 and 31 to go but today is game 133), but with 3 H2H games to play, you are really looking at making up 2 games in the next 26 games, then you at least control your destiny going into the final 3 games. Yes that then requires you to sweep the Brewers which is not at all easy but its in our hands at least.
Also to note, if the Cubs lose today, they will be 4-6 against the Brewers and will need to sweep them to win the H2H tiebreaker. If they win today, they'll only need to win the final series of the season to win the H2H tiebreaker.