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. Highly touted backup Davis Mills completed 7 of 14 passes for 81 yards.
Before leaving, Costello, a second-team All-Pac-12 selection in 2018, completed 16 of 20 passes for 152 yards with a touchdown and no interceptions. Stanford (1-0) travels to USC next week to open Pac-12 play.