O'Sullivan was designated for assignment after his start Sunday, Mass Live reports.
This move was entirely expected. O'Sullivan, who gave up six runs (five earned) on five hits and two walks Sunday, was filling in for a couple of turns through the rotation. With Boston having two off days in the next eight days, they can get by with four starters and bide their time until Eduardo Rodriguez (knee) and/or Joe Kelly (shoulder) is ready for the rotation. Each of those injured pitchers are nearing the end of rehab stints. The Red Sox are expected to call up a position player to take the roster spot O'Sullivan had been occupying.