O'Sullivan allowed six runs on six hits in three innings during a rehab start at Triple-A Pawtucket. He struck out four and didn't walk a batter.
O'Sullivan lasted just 53 pitches before getting yanked. The right-hander, who had a few starts in Boston when the Red Sox had some problems at the back end of the rotation, is expected to be assigned to Pawtucket when he's ready to come off the disabled list.