Bumgarner pitched seven innings in his first start since being injured in a dirt bike accident three months ago. He left with the score tied at 3 after allowing four hits, including home runs by Matt Szczur and Jabari Blash, while striking out five and walking two.
Blash started the winning rally when he singled leading off the ninth against Cory Gearrin (3-3) and advanced on Erick Aybar's sacrifice bunt against Steven Okert. Sanchez then drove a pitch off the Western Metal Supply Co. Building in the left-field corner for the win. It was his fourth. Sanchez started at catcher because Austin Hedges was held out as a precaution after taking a foul ball off his mask on Friday night.
Brandon Maurer (1-4) pitched the ninth for the win.
The Giants lost for the sixth time in eight games. It was San Diego's sixth win in nine games.
Bumgarner was making his first start since April 19 at Kansas City. The next day, he wiped out while riding a dirt bike on a day off in Denver, spraining the AC joint in his left shoulder and suffering bruised ribs.
The lefty retired the first seven batters before Szczur homered to left, his third. Bumgarner walked Carlos Asuaje leading off the fourth and retired the next two batters before Blash drove a two-run shot an estimated 424 feet to straightaway center field for a 3-2 lead. It was his second.
Joe Panik tied it at 3 with a solo shot to right leading off the sixth, his sixth.
Jhoulys Chacin gave up three runs and six hits in six innings, struck out four and walked four.
He labored through a 34-pitch first inning, when he loaded the bases on two walks and a single before allowing a two-run single by Hunter Pence.
Giants: Johnny Cueto was placed on the 10-day DL with blisters on the tips of his right thumb, forefinger and middle finger. Cueto (6-7, 4.59 ERA) left Friday night's game after four innings because of the blisters. He also was affected by blisters on two fingers in May. With Cueto expected to miss at least two starts, Matt Cain moves back into the rotation.
Padres: Hedges was scratched a few hours before the game as a precaution after taking a foul ball off his mask Friday night.
Giants: RHP Jeff Samardzija (4-10, 4.58 ERA) is scheduled to start the series finale. His 10 losses are tied for the most in the NL.
Padres: RHP Trevor Cahill (3-3, 3.38 ERA) is scheduled to make his third start since coming off the DL (strained right shoulder).