Byrd has been with the Indians the past three years. He is 7-10 with a 4.53 ERA this season, but 4-0 since the all-star break.
Boston's pitching staff has suffered from injuries. Curt Schilling had season-ending shoulder surgery, Bartolo Colon has been sidelined since June and Tim Wakefield recently was placed on the 15-day disabled list.
Byrd will start for Boston against Toronto Friday, mlb.com said.
The 13-year veteran has also played for the New York Mets, Atlanta, Philadelphia, Kansas City and Los Angeles Angels.