The Cubs sit 7 1/2 games back in the NL Central standings, and hold the second Wild Card spot with a 9-game lead over the Giants.
Arrieta (18-6, 2.03) will start the first game for the Cubs against lefty Adam Morgan (5-5, 4.42 ERA). Kyle Hendricks (6-6, 4.08 ERA) will face the Phillies' Alec Asher (0-2, 10.61 ERA) in the second game.
Arrieta leads the Major Leagues with 18 wins and is the first Cubs pitcher to reach 18 in a season since Carlos Zambrano went 18-13 in 2007. The last Cubs pitcher with more was Jon Lieber, who went 20-6 in 2001.