O'Sullivan will likely make just one start because the Red Sox have an off-day on Thursday and only need a fifth starter once before the All-Star break.
The Red Sox don't have a lot of internal options at the moment. There was some speculation that Aaron Wilkerson (4-1, 2.20 ERA for Pawtucket) would get a chance.
But the club instead chose a pitcher with Major League experience in O'Sullivan who made three appearances (two starts) earlier this season, going 1-0 with a 7.94 ERA.
"That was the recommendation," said manager John Farrell. "Wilkerson, we recognize, has been throwing the ball well there. The recommendation was for Sean to come back up here and pitch on Sunday."
O'Sullivan, who is out of Minor League options, was designated for assignment by the Red Sox on May 16, but he returned to Pawtucket after clearing waivers.
For Pawtucket this season, O'Sullivan is 6-2 with a 2.79 ERA in 11 starts.
• Hanley Ramirez was out of Friday's lineup two days after exiting Wednesday's game with a strain in his right side. The Red Sox hope to have their first baseman back in the lineup by Saturday.
• Catcher Ryan Hanigan (neck strain) will continue his Minor League rehab assignment by playing nine innings for Pawtucket on Saturday and Sunday. It's possible Hanigan could be activated by the Red Sox on Monday.
• Right-hander Joe Kelly, who was optioned to Pawtucket a month ago, is finally making progress from a right groin strain.
"[He] threw a bullpen on Thursday -- a 20-pitch 'pen after his long toss," said Farrell. "His next scheduled 'pen would be likely tomorrow. There's steps being made. There's going to be the need for probably two additional bullpens before we get him back in a ballgame."
Ian Browne has covered the Red Sox for MLB.com since 2002. Follow him on Twitter @IanMBrowne and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.