An MRI revealed that Cain does not have ligament damage in his right forearm, and that his issue is "more like a two-week thing," Bochy said after the Giants' 5-4 win over the Arizona Diamondbacks.
Cain will be the second Giants starter to be scratched from his opening start, as Jake Peavy won't pitch Tuesday because of back stiffness.
Ryan Vogelsong will start against Arizona on Tuesday, and Tim Lincecum and minor leaguer Chris Heston are possibilities to replace Cain on Wednesday.
"It doesn't sound like it should be major," Cain told reporters. "It sounds like it's just a strain. We'll take it as it is and let it heal up. The trainers seem pretty positive."
Bochy said Cain mentioned tightness in his forearm after his last spring start Friday, when he threw four scoreless innings against the Athletics.
Cain underwent surgery on his right elbow last August to remove bone chips, ending his season.
The 30-year-old right-hander has a 95-95 career record -- including a 16-5 mark in 2012 -- with a 3.39 ERA and 1,505 strikeouts over 10 seasons with the Giants.
Matt Cain not ready to make start for Giants
ESPN SweetSpot blogger David Schoenfield discusses Giants SP Matt Cain's flexor tendon strain.