Cain has been solid since the All-Star break, though, and he'll look to help deal the Milwaukee Brewers a fifth loss in six games as the last-place clubs continue their series Tuesday night.
The Giants (50-61) have won two of the last three World Series, but they currently occupy the bottom of the NL West. Cain (7-6, 4.57 ERA) isn't living up to expectations, either - he's on pace for the worst ERA of his career.
The right-hander, though, has gone 2-0 with a 1.80 ERA in three starts since the All-Star break and has struck out seven in each of the last two. He allowed one run in eight innings of a 2-1 win over Philadelphia on Thursday.
"We just want to go the rest of the season playing well and having a good time," Cain said. "It's no fun when things aren't going well and you're losing."
He's had some disastrous outings this season. Cain allowed seven runs - including homers to Jonathan Lucroy, pitcher Yovani Gallardo and the now-suspended Ryan Braun - in six innings of a 7-2 loss April 18.
Cain is just 3-6 with a 5.05 ERA in 10 starts versus the Brewers, but he has a 3.12 ERA in the four outings in San Francisco. Lucroy is 6 for 15 (.400) with a homer, triple and double lifetime off Cain.
The Brewers' catcher went 2 for 4 in Monday's opener of a four-game series, but San Francisco scored three in the eighth and won 4-2. Brandon Belt, who was 3 for 17 over his previous seven games, went 3 for 3 in the victory.
Jeff Francoeur drove in the go-ahead run with a single in the eighth, though the Giants finished 1 for 13 with runners in scoring position.
''We didn't get anything going until the last inning,'' manager Bruce Bochy said. ''We needed something.''
Milwaukee (47-65), which is last in the NL Central, has been held to two runs or fewer in each of its four losses over the last five games. Juan Francisco drove in both runs Monday with a seventh-inning double and ninth-inning homer.
Leadoff hitter Norichika Aoki went hitless and is 2 for 25 (.080) in his last eight.
''We had opportunities and we didn't come through,'' manager Ron Roenicke said.
The Brewers will look to bounce back with help from Wily Peralta, who is mired in a personal rut.
Peralta (7-11, 4.57) went 2-0 with a 0.31 ERA as Milwaukee won his first four starts of July, but he allowed eight runs - five earned - in 3 2-3 innings of an 8-3 loss to Colorado on July 26. He then yielded four runs in five frames of Wednesday's 6-1 loss to the Chicago Cubs.
"When he puts the ball down the middle of the plate, I don't care if you throw 100, you're going to get hit," Roenicke said.
The right-hander didn't have good stuff April 16 when he faced the Giants, either, giving up six runs in four innings, but the Milwaukee offense backed him its 10-8 win. Brandon Crawford hit a solo homer while Pablo Sandoval had a two-run double and an RBI single off Peralta in that contest.
The Brewers have lost five of their last six in San Francisco.