The Giants won that game at Coors Field, 9-8, and even though Bumgarner did not get credit for the pitching victory, he helped his team by hitting a three-run homer off Chacin in the fourth inning.
After a 1-2-3 first inning, Bumgarner struggled in the second and third, allowing four runs on seven hits and a hit batsman to fall behind 4-0.
The fourth inning was Chacin's last inning because the Rockies have imposed a 75-pitch limit on their starting pitchers and he could not hold the lead.
"[Bumgarner] got us back in the game with the home run," Giants manager Bruce Bochy said.
Chacin, obviously, was disappointed at giving up the home run to Bumgarner.
"I was trying to throw a fastball down and away, and that ball just ran back to the middle," Chacin said. "I know he's a good hitter. In the big leagues, whoever is hitting, if you miss your spot, you make your mistake, they'll take advantage of that. It was a really bad pitch to him."
Bumgarner is having a good season with a 14-10 record and 3.32 ERA for the National League West-leading Giants.
But it was Bumgarner's fourth consecutive outing of allowing four or more runs in less than seven innings and only his third outing under five innings this season.
"The ball was up a lot. That didn't work out too good," said Bumgarner. "I didn't have very good command of really any pitch away. I pitch in a lot, but you got to be able to throw away, too, and I wasn't able to do that. It seemed like they were just sitting on one spot."
Chacin is 2-5 with a 4.86 ERA.
Rockies: Ottavino has meltdown
Shortstop Josh Rutledge hit a grand slam and starting pitcher Alex White hit a three-run shot in Sunday's 12-11 loss to the Padres, but Adam Ottavino, who relieved Witt to start the fifth inning, gave up seven earned runs in two-thirds of an inning.
Giants: Pagan breaks San Francisco record
Angel Pagan hit his 13th triple of the season in Saturday's game against the D-backs, the most any Giant has amassed since the franchise moved to San Francisco in 1958.
Pagan had shared the record with Willie Mays (1960) and Steve Finley (2006).
The Giants lost to the D-backs, 10-2, Sunday. It was the most runs the Giants have allowed since Washington beat them 14-2 Aug. 13.