Joe Kelly has been optioned down to AAA Pawtucket following a terrible first few months of pitching this season. As a result, Justin Masterson will make Kelly's scheduled Sunday start.
Kelly had pitched to a 5.67 ERA and 1.50 WHIP this season, so the move makes sense. It's not like Masterson has been good (6.37 ERA) this year, but at the very least, the Red Sox are shaking things up.
Source: Nick Cafardo via Twitter