Despite the injury, there's still a chance that Guthrie could make his next scheduled start Tuesday.
Guthrie left Thursday's game in Toronto after feeling stiffness in his back. The results of an MRI Friday were "as good as one could have hoped," president of baseball operations Andy MacPhail said.
MacPhail says Guthrie could start Tuesday in Pittsburgh next week, depending on how the pitcher responds to treatment over the next two days.
Guthrie is 2-8 with a 3.56 ERA. He has made at least 30 starts in each of the past four seasons.