Bumgarner ranks fourth in the majors with a 2.15 ERA since June 11, and Chacin is fifth at 2.26. Bumgarner (11-8, 2.84 ERA), though, has been plagued by poor run support - the Giants have backed him with only 10 runs while he's gone 1-3 with a 2.35 ERA in his last seven starts.
The left-hander's lone mistake in eight innings Friday was a costly one - a three-run homer by Clint Barmes in a 3-1 loss to Pittsburgh.
"I thought I made a good pitch," said Bumgarner, whose run-support average of 3.63 per start is among the lowest in the majors. "It was pretty much where I wanted it. He just made an adjustment."
The homer also went against form for Bumgarner, whose .200 batting average by opponents with runners on base is one of the best in the league. He is 7-1 with a 2.95 ERA in his last nine starts against Colorado, with the only loss coming May 17, when he allowed nine runs - seven earned - in 4 2-3 innings of a 10-9 defeat.
Bumgarner bounced back with seven innings of one-run ball in a 5-2 win on June 30, yielding only a homer to Carlos Gonzalez - currently on the disabled list with a sprained finger - and three other hits.
He has pitched at least seven innings in 11 of his last 13 starts.
Chacin (12-7, 3.22), meanwhile, is coming off a 3-2 win Friday in Miami, where he allowed two runs and four hits in seven innings. It was an emotional victory after spending the week leading up to his start in his native Venezuela for his grandmother's funeral.
"I was real excited to pitch," said Chacin after the Rockies (62-72) rallied to give him his career-best 12th victory. "It was a tough situation back home, but when I got here I wanted to have all of my concentration to try to throw strikes and a lot of innings."
Chacin has lasted at least seven innings in six of his last seven starts. The right-hander has been hit-and-miss in two starts versus the Giants this year, scattering three hits and three walks in eight shutout innings of a 4-1 win June 28, but getting tagged for a season-worst eight runs in 5 1-3 innings of an 8-6 defeat May 13.
Pablo Sandoval, 7 for 21 lifetime off Chacin with four extra-base hits, helped the Giants earn their third win in four games Tuesday with a first-inning solo homer among his three hits. It was his first home run since July 11, ending a string of 139 at-bats without one.
Brandon Belt, with four hits Tuesday, is batting .361 in his last 21 games. After entering this year 0 for 10 versus Chacin, he has two hits in five at-bats.
The Rockies, who are 10-11 since Gonzalez got hurt, have lost seven of 11 overall. Center fielder Dexter Fowler is expected to miss his second straight game with a bruised knee suffered Monday trying to steal second base.