Manager Bruce Bochy had spoken to Bumgarner about getting the Game 2 start for the last week, but Bumgarner didn't get the official word until Thursday. Bumgarner went 16-11 with a 3.37 ERA in 32 starts this season.
"This is what you work so hard in the offseason for, and you get a chance to have it all pay off," Bumgarner said after throwing a bullpen session during Thursday's voluntary workout.
The promising young left-hander has a veteran's worth of postseason experience already, having played a prominent role in the Giants' 2010 World Series run, including throwing eight shutout innings against the Rangers in Game 4 of the World Series.
This time around, Bumgarner will join Cain in being tasked with defending the Giants' home field under this year's Division Series setup, in which the lower seed hosts the first two games.
"If you look at the years that Matt and Madison have had, that's why they're going Game 1 and Game 2 right now, and that's usually how you do it," Bochy said. "Sure, sometimes you make adjustments when you're going on the road, but we feel like these two have earned these starts, and that's why they're going out there."
Bumgarner will be opposed by the Reds' Bronson Arroyo after Cain duels Johnny Cueto in Game 1. The last time Bumgarner faced the Reds, he tossed a one-hitter for his first career complete game.
"This is the postseason. You can't pay attention to what happened in the regular season, whether it was good or bad," Bumgarner said. "You have to go out there and try to make pitches every time and do what you've got to do. That game, fortunately, I was able to do that. I had some things going my way, and you have to just go out there and try to compete and keep us in the game."