Bumgarner hasn't made a relief appearance since throwing two scoreless innings in Game 6 of the 2010 National League Championship Series.
The Giants will start Jake Peavy against the Royals in Game 6. San Francisco holds a 3-2 edge and is one win from its third title in five years.
"Well, I expect Jake Peavy to throw a shutout," Bumgarner told CSNBayArea.com. "That's what I expect. But we'll be ready for anything."
Bumgarner, who threw 117 pitches Sunday, wouldn't estimate how long he could go.
"I'm not a big pitch-count guy," he said. "So as long as you keep getting outs and you feel good, you should stay out there."
"He really bounces back well, and we wouldn't ask him to do a lot," manager Bruce Bochy told CSNBayArea.com. "But if I needed to get an out or something, I'm sure he'll say he's available."
Bumgarner is 4-0 in World Series play with a record-low 0.29 ERA.
Peavy, 33, will face 23-year-old rookie Yordano Ventura in a rematch of Game 2 starters. Ventura picked up a no-decision in that game, while Peavy took the loss.
The veteran is seeking his first World Series win -- he didn't get a decision for Boston in Game 3 last year -- and is well aware he could get the victory in the clincher.
"I can't imagine anything being any sweeter than that," Peavy said. "This is the start that you play your whole career wanting."
In the 41 previous instances in which the Series was 2-2 in the best-of-seven format, the Game 5 winner took the title 27 times. But eight of the past 10 teams to come home trailing 3-2 swept Games 6 and 7.
The Associated Press contributed to this report.