Miller left Ohio State's game early in the first quarter after suffering an injury on a run. He was then carted to the locker room for further evaluation before ESPN reported that he had suffered a slight sprain of his MCL.
According to Ohio State coach Urban Meyer, Miller could play through the injury if he needed to, but the team had decided to let him sit out the rest of the game. A decision that was certainly understandable considering Ohio State already had a 21-0 lead on San Diego State before getting the diagnosis on Miller's injury.
As of publication the Buckeyes have extended that lead to 35-0 as backup quarterback Kenny Guiton has filled in quite nicely for Miller. Guiton has thrown for 102 yards and rushed for 68 more while scoring two touchdowns in Miller's absence.