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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Danny Duffy ($9,600) faces an Athletics offense that ranks in the bottom 6 against lefties this year with an 85 wRC+, and they're ranked as the 5th least productive offense in the second half with an 84 wRC+. They don't strike out much with a K% just under 19% vs lefties, but they've largely been punchless. Duffy has a 3.13 ERA, 3.42 FIP and 27% strikeout rate and should be a good bet to prevent runs and be in decent position to get the win against a team with the 5th worst run differential in baseball.
The Diamondbacks, who rank as the #1 offense vs LHP by OPS at .816, face the well below average Jorge de la Rosa in very hitter friendly Chase Field. de la Rosa has a 5 ERA, 5 FIP and is allowing a .276/.356/.460 line to righties with a similar road slash of .284/.357/.437 to righties, so it isn't entirely Coors Field related.
Paul Goldschmidt ($3,700) has been the best Diamondbacks hitter vs lefties this year with a 176 wRC+ and .239 ISO and has a history of being one of the biggest lefty mashers in the game over the last few seasons. Catcher Welington Castillo ($2,900) has hit lefties to a way above average level for catchers this year with a 142 wRC+ and .272 ISO. Yasmany Tomas ($2,600) has hit LHP to a 175 wRC+ and .336 ISO. Rickie Weeks ($2,300) has hit LHP to a 167 wRC+ and .369 ISO in about 50 less PAs than the others. This stack is relatively inexpensive with 3 of the 4 batters priced under 3k and leaves some room to fit in a higher priced pitcher like Duffy into a lineup.