Stanford senior quarterback K.J. Costello did not return after taking a hit to the head near the end of the first half of the Cardinal's 17-7 win against Northwestern.
The injury occurred on a play during which Costello went into a sliding motion and received a forearm to the head from Northwestern defensive lineman Earnest Brown. It did not draw a penalty for targeting from the Big Ten officiating crew.
The severity of Costello's injury is not clear.
With Costello sidelined in the second half, Stanford did not score any points on offense.