The left-hander gets the nod over righty Tanner Roark, who was slated to go in Game 4 before being replaced by the team's co-ace, Stephen Strasburg. Strasburg had initially been ruled out due to an illness.
The 32-year-old veteran went 15-9 with a 2.96 ERA in 32 regular-season starts.
The Cubs had already announced Kyle Hendricks as their starter, and with good reason. The righty stymied the Nationals in Game 1, allowing only two hits across seven scoreless innings.
Whoever wins will move on to face the Los Angeles Dodgers in the NLCS. Game 1 is slated for Saturday, Oct. 14 at 8:08 p.m. ET.