Cain, 29, is 0-3 with a 4.35 ERA in five starts and 31 innings this season. This is only his second career DL stint -- he missed the minimum 15 days after taking a comebacker to the forearm last August.
The Giants will recall right-hander Jake Dunning to fill the roster spot. The 25-year-old had a 2.84 ERA in 25 1/3 relief innings for San Francisco last season.
In all likelihood, long reliever Yusmeiro Petit will replace Cain in the rotation for the time being. The 29-year-old has a 2.91 ERA in 20 2/3 innings this year.