O'Sullivan's contract was purchased from Triple-A Pawtucket, MLB.com reports. He'll get the start Sunday against the Angels.
The Red Sox have been able to get by with a four-man rotation of late due to scheduling quirks, but with the team in the midst of a stretch of six games in as many days, O'Sullivan was needed to make a spot start. The team could surely use O'Sullivan to eat some innings after starter Clay Buchholz and the bullpen were pummeled for 21 runs in Saturday's loss, but don't count on pretty numbers from the right-hander, who has surrendered 10 earned runs on 19 hits and two walks over 11.1 innings with the Red Sox this season.