He has typically been plagued by the same issue against the Arizona Diamondbacks.
Bumgarner goes for his first victory in over a month Sunday as the Giants try to salvage a four-game split against the visiting Diamondbacks.
Among the NL leaders in ERA and opponents' batting average (.204), Bumgarner (11-9, 2.91 ERA) has missed out on his share of wins thanks in part to a lousy 3.55 run support average. The left-hander has seen things get worse during his last six outings, a stretch during which he has been backed by a combined nine runs.
Bumgarner yielded two runs in six innings but didn't factor in the decision of Tuesday's 3-2 loss at San Diego. He is 0-8 with a 3.02 ERA when receiving two runs of support or fewer compared to 11-1 with a 2.79 ERA when he gets three or more.
"He's as tough a competitor as you can find," said manager Bruce Bochy, whose team has dropped eight of Bumgarner's nine starts since the All-Star break despite his 2.67 ERA.
"He's disappointed not to win, but he goes back out there and gives it all he has."
Bumgarner has been nearly untouchable in four outings against the Diamondbacks, going 1-1 with a 1.71 ERA. While he received a season-high nine runs of support in a 10-5 road victory June 8, the Giants have provided him with a combined one in his other three starts in the series.
Bumgarner will try to help San Francisco (63-79) get back on track following Saturday's 2-1 loss. Matt Cain pitched effectively into the seventh inning in his return from the disabled list, but the Giants were limited to six singles.
San Francisco has averaged a major league-worst 2.45 runs at home since July 7, going 10-21.
Winner in three of four, Arizona (72-69) is eight games behind Cincinnati for the NL's second wild-card spot.
The Diamondbacks give the ball to Wade Miley (9-10, 3.78), who is 0-2 with a 4.85 ERA in his last five outings. The left-hander is coming off the shortest start of his career, giving up five runs and eight hits before exiting with two outs in the second inning of Tuesday's 10-4 loss to Toronto.
"I got a little upset with myself," he told MLB's official website. "Just the fact that I lost it like that. ... I feel like I've had that same exact stuff and thrown seven innings and given up a couple of runs and given us a chance to win.
"Just one of those games you've got to put it behind you and move forward."
Miley is 0-3 with a 4.15 ERA in four career starts versus San Francisco. While he has had a rough time against Buster Posey (6 for 12 with a homer and two doubles), the All-Star catcher is likely to miss a fifth consecutive game due to a swollen right ring finger.
Arizona's Martin Prado is uncertain to be available after sitting out Saturday with an illness.