Cain was set to take the hill for the first time in almost two weeks after being scratched Tuesday with a cut on the tip of his right index finger, which he suffered while attempting to pick up a knife. The Giants, though, scuttled those plans Sunday night and placed Cain on the 15-day disabled list, retroactive to April 24.
"It's on the forefinger about half an inch, but it's in a bad spot there," manager Bruce Bochy said.
Cain has been superb against Pittsburgh of late, going 3-0 with a 1.72 ERA during a seven-start stretch in the series, but San Francisco has reason to be confident with Petit taking the ball after a strong performance in Cain's place Tuesday. The right-hander gave up three hits and struck out four with no walks in six innings of a 6-0 victory over San Diego, his first start since Sept. 28.
Petit also dominated the Pirates in a 6-0 win in his last start against them, giving up one hit in eight innings at Pittsburgh on Aug. 4, 2009.
The Pirates' Jeff Locke is expected to be recalled for a spot start following Thursday's doubleheader. The left-hander made the All-Star team last year before struggling down the stretch, finishing 10-7 with a 3.52 ERA.
Locke went 1-5 with a 6.66 ERA over his final 11 starts and was left off the postseason roster.
"He's certainly had his share (of challenges) since the second half of last season," manager Clint Hurdle told the Pirates' official website. "This is a great opportunity for him to embrace what he has and to keep focused on where he wants to go, and how to get there."
Locke was 0-2 with a 5.04 ERA in five starts with Triple-A Indianapolis. He allowed three runs, five hits and four walks in four innings of a 10-5 loss at San Francisco on Aug. 22 in his only career start against the Giants.
Locke will try to shut down a San Francisco club that's won five straight and nine of 10. The Giants (20-11) opened a 10-game trip with a three-game sweep of Atlanta - their first road sweep in the series since 1988.
Brandon Crawford did most of the damage with a pair of homers in Sunday's 4-1 win, and Madison Bumgarner threw six strong innings. The Giants' rotation has allowed four earned runs during the winning streak while the bullpen has thrown 12 1-3 scoreless innings.
"That's incredible," Bochy said. "What a job our pitching did here."
The Pirates had lost 15 of 19 before posting back-to-back wins over Toronto, but their struggles resumed in a 7-2 defeat Sunday. Pittsburgh, which managed four hits, has dropped 13 straight when scoring three or fewer runs.