Reminder: Always double check the lineup card before rosters lock to make sure that a player you've chosen to roster is playing that day, and check the weather to make sure your players won't get rained out of their game.
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I'll give one "safe" play and one "risky" play here.
The safe, but more salary restrictive play is to use Jeff Samardzija ($9,900) against a rotten Padres offense that ranks last in wRC+ vs RHP (71), fourth from last in ISO vs RHP (.128) and has the 4th highest K% vs RHP at 25.2%. Samardzija has a 2.88 ERA, 3.06 FIP and 21% K% this year and is pitching much better with his new team and run scoring environment. He is a good bet to prevent runs, get strikeouts and be in position to get the win.
The risky, but more cap friendly play is to use Jon Gray ($6,900) on the road against the Cardinals. Here's the positive side: Gray has an elite 30.5% strikeout rate, a 2.34 FIP and has pitched extremely well on the road away from Coors, although in a minuscule sample size. I also watched Gray pitch last Friday in Coors and was impressed with him. His stuff was electric, and he got some good bite on breaking pitches, which is difficult to do in Coors Field with the air affecting pitch break. His stuff looked much better than it did last year when I watched him. His fastball has added 1 mph in velocity, and he's throwing a good curve.
The risk here is that the Cardinals offense has the highest ISO (.215) and 2nd highest wRC+ (124) vs RHP this year, and Gray has a 4.71 ERA. He also opposes Michael Wacha, which will make run support less likely. But his skill level appears to be closer to his 2.34 FIP than his 4.71 ERA, especially when he's used only on the road away from Coors and in a much more neutral run scoring environment. Because his ERA is so poor, I expect him to be lower owned in tournaments. I wouldn't use Gray in a head to head matchup, but he could pay off in a tournament by aiding in salary relief and possessing the upside to rack up a big strikeout total.