Heston, who was sent to Triple-A to clear roster room last week for right-hander Mike Leake and to receive a break from the physical stress of his first full Major League season, will inherit Cain's start in Sunday's series finale against St. Louis.
Cain, 31, went on the disabled list for the fifth time in two years. He expressed hopes of being ready to pitch when he's eligible to be reinstated Sept. 10. Whenever he returns, Cain, who has started in all but one of his 291 Major League appearances, said he'd be willing to work out of the bullpen if a starting spot is not readily available.
"Whatever it takes to get into the postseason, I'm up for it," said Cain, who's 2-4 with a 6.15 ERA in 10 starts.
Manager Bruce Bochy refused to speculate about Cain's possible role.
"It's hard for me to answer that right now," Bochy said, noting he can address that issue more easily as Cain's activation draws closer.
A source of Cain's optimism was the MRI he underwent Wednesday that revealed no structural damage.
"That was really nice," said Cain, who also received a cortisone shot.
Cain described his discomfort as an "achy" feeling that he experienced at various junctures of the day. This contributed to his inability to maintain a consistent arm slot, which in turn robbed his pitches of movement and made them easier to hit.
Cain, a three-time All-Star, underwent surgery last August to remove bone spurs and chips from his elbow, then went on the DL this year with a flexor tendon injury that prevented him making his season debut until July 2.
Heston, 11-7 with a 3.34 ERA, has pitched 148 2/3 innings, a number he has matched or exceeded in four of the previous five seasons. But the Giants sensed he would benefit from a brief rest. Therefore, Heston did not pitch at Triple-A and limited his activity to a side session or two off a bullpen mound.