Masterson's contract for one season, as he tries to rebuild his value after having the worst season of his career in 2014.
Since losing Jon Lester to the Cubs on Tuesday night, the Red Sox sent slugger Yoenis Cespedes to the Tigers for Rick Porcello, while they are also close to a deal that will bring lefty Wade Miley on board for Rubby De La Rosa, Allen Webster and a Minor Leaguer.
Masterson, 29, was selected by the Red Sox in the second round of the 2006 First-Year Player Draft. He was promoted to the Major Leagues by Boston in 2008 and served as a swing man for the club until being dealt to the Indians for Victor Martinez on July 31, 2009.
Masterson went on to have some success in Cleveland, most notably in 2013, when he went 14-10 with a 3.45 ERA. The Indians traded Masterson to the Cardinals in July, but his season-long slump only worsened in St. Louis.
A rib-cage injury that Masterson tried to pitch through could have contributed to the down year he had in 2014.
Once all of Boston's new moves become official, the projected rotation will consist of Clay Buchholz, Joe Kelly, Porcello, Miley and Masterson.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.