The Cubs sit 7 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.
Arrieta is 12-1 with a 0.99 ERA and 108 strikeouts in his last 15 starts, all quality starts, dating to June 21 in Minnesota. That one loss was against the Phillies on July 25 when Cole Hamels threw a no-hitter at Wrigley Field.
Arrieta's 2.03 ERA and .194 opponent batting average are second to the Dodgers' Zack Greinke (1.68 and .192) in the Major Leagues, and the Cubs' right-hander ranks fourth in the NL with 197 strikeouts. He needs three strikeouts to become the first Cubs pitcher to reach 200 K's in a season since Jeff Samardzija fanned 214 in 2013.
The Cubs tweaked their rotation for the remainder of the Phillies series. Dan Haren (9-9, 3.73 ERA) will start on Saturday and Sunday's starter is to be determined, and may be a "bullpen" day, manager Joe Maddon said. That would line up Jon Lester and Jason Hammel for Tuesday's doubleheader against the Pirates.
Why did the Cubs and Phillies wait so long to call Thursday's game?
"There were conflicting reports," Maddon said.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings. You can follow her on Twitter @CarrieMuskat and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.