Hernandez felt the twinge after slipping while making a pitch in the second inning of Friday night's 7-4 loss to the Chicago White Sox.
He remained in the game for three more innings. He gave up three home runs overall and issued four walks in a no-decision.
Hernandez said Saturday his back is still a little sore.
Hernandez, the 2010 Cy Young winner, is 4-4 with a 3.42 ERA this season. He's 1-3 with a 6.14 ERA in his last five starts.
Mariners manager Eric Wedge said Hernandez could have stayed in the game, and wanted to remain in. Wedge said he saw no sense in pushing it.