The San Francisco Giants acquired right-hander Jake Peavy from the Boston Red Sox on Saturday for a pair of minor league pitchers.
He will start Sunday night's series finale against the Los Angeles Dodgers. The Giants held a half-game division lead in the NL West against their rival going into Saturday night's middle game at AT&T Park.
San Francisco is sending right-hander Heath Hembree and lefty Edwin Escobar to the World Series champion Red Sox in the swap.
Peavy (1-9, 4.72 ERA) has been hurt by poor run support this season, receiving two runs or fewer of support in each of his past eight starts.
Yusmeiro Petit had been scheduled to pitch Sunday for the Giants with right-hander Matt Cain on the disabled list.
The Giants made the deal without having to give up their top pitching prospect, Kyle Crick.
It's been a busy week for Giants general manager Brian Sabean and assistant GM Bobby Evans.
On Friday, they promoted second baseman Dan Uggla from Triple-A Fresno only four days after they signed him to a minor league contract Monday. Uggla was released by Atlanta on July 18.