Stanford QB K.J. Costello Out vs. USC with Head Injury; Davis Mills to Start

Stanford Cardinal quarterback K.J. Costello has been ruled out Saturday's showdown with the USC Trojans due to a head injury. 

Junior Davis Mills is expected to start for the first time in his career in place of Costello.

Costello led Stanford to a 17-7 victory over the Northwestern Wildcats in Week 1, as he completed 16 of 20 pass attempts for 152 yards and one touchdown. However, he left the game at the end of the first half after taking a late hit to the helmet from Northwestern defensive end Earnest Brown IV.

Mills replaced Costello and completed seven of 14 pass attempts for 81 yards.

"We'll see how his injury progresses during the course of the week," Stanford coach David Shaw said Tuesday, per the Mercury News' Vytas Mazeika. "Should hopefully make a decision by Thursday or Friday."

Costello has served as the Cardinal's starting quarterback since the middle of the 2017 season. He has made  21 starts over the past two-plus seasons. The senior was named to the preseason Davey O'Brien Award watch list.

With Costello entrenched as the starting quarterback, Mills has seen very limited action during his time at Stanford. Including his relief appearance against Northwestern, the 6'4", 214-pound signal-caller has played in just two games for the Cardinal.

He has just 16 career pass attempts and has yet to throw a touchdown pass.

Even though Mills does not have much experience at the collegiate level, Shaw will not water down the game plan.

"Whoever starts at quarterback, it’s not going to change our philosophy, it’s not going to change our scheme," Shaw said Tuesday, per Mazeika. "All of our quarterbacks know our offense and now it’s just about getting ready to play."

Saturday's clash at the L.A. Memorial Coliseum will feature a pair of backups, as the Trojans will also be without their starter. J.T. Daniels is out for the season after suffering a torn ACL and meniscus in a season-opening win over Fresno State on Saturday, elevating true freshman Kedon Slovis to starter.