Manager Alex Cora said Jansen tweaked his lower back during Wednesday's game against the Mariners, Mac Cerullo of the Boston Herald reports.
Jansen appeared to be hurting on the mound during the ninth frame of Wednesday's win, though he remained in the game and finished the inning after Boston's training staff checked on him. He'll get an extra day to rest during the Red Sox's day off Thursday, and he'll be re-evaluated before Friday's game against Texas.