No. 23 Stanford announced Thursday that quarterback K.J. Costello has been ruled out of Saturday's game vs. USC. Junior Davis Mills is expected to make his first career start in his place. 

Costello took a forearm to the face from Northwestern's Earnest Brown, and his head hit the turf hard late in the first half in Saturday's win over the Wildcats. Brown was not ejected for targeting on the play, even though Costello was giving himself up on the play as Brown dove at his chin. He was 16-of-20 for 152 yards and one touchdown in the first half prior to the injury. 

Costello was named to the second team of the preseason All Pac-12 team by the media. He threw for 3,540 yards and 29 touchdowns as a sophomore in 2018.

Mills took over for Costello and went 7-of-14 passing for 81 yards in the second half, helping the Cardinal notch a 17-7 win. He was a five-star prospect and the No. 15 overall player in the Class of 2017 out of Greater Atlanta Christian School in Duluth, Georgia. Despite the high school hype, the 6-foot-4, 214-pounder has played in just two games during his Stanford career. He was 0 of 2 against UC-Davis in September 2018 in his only game action prior to last Saturday. 

The 247Sports scouting report on Mills provides a good glimpse of what kind of player he is.

"Mills is a very efficient passer. His passes are very catchable, he throws with good timing, and he just knows where to put the football. He can force some balls at times, but he will improve with that over time. He has a nice release, the ball comes out spinning tight, and he throws with very good accuracy. He really plays the game at a good pace and never seems overwhelmed. Smart and aware with good command as well."

It will be a battle of backup quarterbacks on Saturday night. Kedon Slovis will start for USC after J.T. Daniels tore his ACL in the Trojans' season opener vs. Fresno State. Kickoff in the Pac-12 opener for both teams is set for 10:30 p.m. ET from the Los Angeles Memorial Coliseum.