I know that the official word is "back injury", but I firmly believe he reinjured his throwing shoulder - not his back - vs MSU. He was driven into the turf squarely on his shoulder, same one injured last year and operated on after the NCG. Ideal hit to reinjure the shoulder.
He's been much more inaccurate ever since. 68.9% pass completion before injury, 57.4% since. 9.5yds per attempt before injury, 7.8yds per attempt since. I think knowing he's injured, opponents have been stacking the box to try to stuff the run and force us to throw.
Of course as the offense has struggled to maintain ball-control, the defense is less rested.