The Cardinals hitters are very disciplined and will look to wear Arrieta down by making him go deep into counts. If they can successfully do so and get Arrieta out of the game by the 7th or 8th, that could mean you are trusting the Cubs bullpen. Which can be a scary thing for under bettors.
Wacha has not faired well against the Cubs and has been less than stellar down the stretch. The Cubs could very well put up a six spot against him.
I am not trying to talk you out of it, in fact I tend to agree it's the right side here. Just a few things to consider before you unload your bankroll.