Guerrero's (shoulder) stay on the injured list has been extended due to lower-back soreness, Chris Cotillo of MassLive.com reports.
Guerrero was trending toward a minimum-length absence after landing on the injured list Sept. 1 with shoulder inflammation, but a back injury will delay his return. The team doesn't seem to be very concerned about the 35-year-old's new injury, however, so he'll likely still be able to rejoin Boston's bullpen before the end of the season.