O'Sullivan is the expected starter for Tuesday night's game against the Athletics, the Boston Globe reports.
Manager John Farrell said "everything points to" the right-hander, who will be filling in for the demoted Henry Owens. This is considered a one-time-only gig, as Eduardo Rodriguez (knee), who pitched a third rehab start Sunday, is due back during this next turn through Boston's rotation. O'Sullivan has been pretty good in five starts at Triple-A Pawtucket in 2016, but his body of the work in MLB as a starter (6.07 ERA, .304 BAA) is not optimistic.