Jack Flaherty and Lucas Giolito are heavyweight pitchers. But I'm siding with Flaherty especially in the rare role as an underdog. 
Flaherty is a solid Cy Young candidate being 8-0 with a 2.53 ERA and 0.99 WHIP. The White Sox have never faced him putting them at a disadvantage. 

The White Sox dominated against lefty starters, but are just 17-16 versus righthanded starters. 

Giolito has a 4.35 ERA this season, although he has been pitching better. This will be his third career start against St. Louis. He's allowed eight runs of which seven were earned in 11 1/3 innings against the Cardinals. 

Giolito has to deal with a hot Nolan Arenado, who has four homers and eight RBI's during his last 10 games. He's 3-for-3 lifetime lifetime versus Giolito.