Breslow was a key part of the Boston's bullpen in 2013, when the Red Sox won the World Series. But he struggled in the Series and again this year, when he had a 2-4 record and 5.96 ERA.
General manager Ben Cherington indicated Thursday he was open to bringing back the 34-year-old Yale graduate. But by declining the option, which triggered a $100,000 buyout, the Red Sox allowed Breslow to become eligible for free agency.
Also Friday, the Red Sox reinstated outfielder Shane Victorino from the 60-day disabled list. He had back surgery on Aug. 5.