The Red Sox designated Masterson for assignment after Sunday's 7-2 win over Detroit.
Masterson is 4-2 with a 5.61 ERA in 18 appearances, including nine starts.
Manager John Farrell said the Red Sox will use Masterson's roster spot to give innings to younger pitchers.
''It was consistent strike-throwing, when you boil it down to it,'' Farrell said. ''He flashed some better arm strength at various points in time this season, but the overall consistency just wasn't there.''
The Red Sox also got Triple-A third baseman Carlos Rivero from Seattle for cash.
Rivero played four games for Boston last year, then signed a minor league free agent deal with the Mariners.
Rivero was hitting .256 with eight home runs and 40 RBIs for Triple-A Tacoma. He will be assigned to Triple-A Pawtucket.