Enduring a strained flexor tendon in his throwing arm, Cain all but officially has been removed from his turn in Wednesday's series finale. The ailment could sideline Cain, who underwent an MRI to confirm the diagnosis, for two weeks.
"It's definitely frustrating, but it kind of goes with the territory," said Cain, who needed season-ending elbow surgery to remove bone chips and spurs last August. "Sometimes there are going to be bumps in the rehab process, and this is one of them."
As possible replacements for Cain, Giants manager Bruce Bochy named Tim Lincecum, who's tentatively scheduled to start Friday in San Diego, or a Minor Leaguer. Right-hander Chris Heston's impressive spring performance makes him a candidate for promotion.
Bochy acknowledged that "there has to be some concern" about Cain, 30.
Bochy said he expected Peavy to return sometime during the weekend in San Diego. The 33-year-old, who signed a two-year, $24 million contract last December, is coming off a Spring Training in which he recorded a 9.64 ERA in six starts.
Bochy said he preferred to leave Yusmeiro Petit in his long-relief role and declined to include him in the mix of potential starters.
Given the rotation's shaky health overall, the Giants' decision to open the season with 13 pitchers, one more than usual, seems prudent.
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, and follow him on Twitter at @sfgiantsbeat. This story was not subject to the approval of Major League Baseball or its clubs.