Jansen will not be available to pitch Sunday due to back tightness, Christopher Smith of MassLive.com reports.
Jansen pitched through a back issue that cropped up prior to Saturday's game and was visited by training staff during the ninth inning. "He'll be down tomorrow," Boston manager Alex Cora said. "Hopefully, he'll be back on Monday." If a save situation arises Sunday, Cora could use a returning Chris Martin (shoulder), who's expected to come off the IL before the game, while lefty Richard Bleier and righty John Schreiber are also available. Ryan Brasier, who picked up a save earlier this season, is also an option, but the right-hander threw 21 pitches Saturday and has allowed eight runs over his last 4.2 innings.