"Hopefully it is not a big deal," Cain said after the Giants' 6-0 win. "Petit did a great job. He gave us a lift. To have a guy like him in the bullpen is a huge asset."
Manager Bruce Bochy said it was not a bad cut, but it was in a bad spot on his finger.
"It should close up in a couple of days," Bochy said. "He should be fine."
Cain said he was cutting a sandwich when the knife fell and he grabbed at it before it hit the floor. He jokingly tried to blame the injury on left-hander Jeremy Affeldt, who sustained his own knife-cutting incident a few years ago.
"He had a lot of fun with it," Cain said. "Considering how much I let him have it, it's my turn to feel like an idiot."
Affeldt said Cain "should feel like that. He made fun of me for three years."
Cain had not missed a start in his career until last season. His 184 1/3 innings in 2013 marked the first time he didn't throw at least 217 2/3 innings since throwing 190 2/3 in his 2006 rookie campaign.
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.