Saturday night’s Stanford-USC game — which a week ago was expected to feature a matchup of top-flight Pac-12 quarterbacks — might be a matchup of backups.

USC quarterback JT Daniels will miss the rest of the season after sustaining a torn ACL and meniscus during Saturday’s 31-23 victory over Fresno State. That means true freshman Kedon Slovis will be USC’s starting quarterback against the No. 23 Cardinal in both teams’ conference opener in Los Angeles.

Meanwhile, Stanford head coach David Shaw said quarterback K.J. Costello is questionable after being knocked out of Saturday’s 17-7 victory over Northwestern after taking a blow to the head. Shaw was critical of the officiating on the play, which resulted in a personal foul against Northwestern’s Earnest Brown IV but not an ejection.

“I believe the officials on the field and the replay officials missed the call,” Shaw said. “I thought it was an ejectable offense. I’m not going to comment anymore on that other than to say they missed the call.”

Shaw’s immediate concern is Costello, a second-team All-Pac-12 selection last year.

“We’ll see how his injury progresses throughout the course of the week,” Shaw said Tuesday. “We should hopefully make a decision by Thursday or Friday. We should know before game day.”

The public might not know before game day, because Stanford coaches and players are not available to the media after Wednesday.

If Costello can’t play, Stanford would go with sophomore Davis Mills, who completed 7 of 14 passes for 81 yards and no touchdowns in relief of Costello. Mills had thrown just two passes (both incomplete) before Saturday.

His task is made more difficult by the fact that Stanford’s all-conference left tackle Walker Little sustained a lower body injury that will sideline him for several games.

“There’s still a good chance we’ll get him back, hopefully by midseason,” Shaw said.

But instead of having an Associated Press preseason first-team All-American protecting the blind side of the Stanford quarterback on Saturday, Costello or Mills will have true freshman Walter Rouse starting at left tackle.

Mills was a highly touted quarterback coming out of high school before injuries slowed his progress. Scout and Rivals both rated Mills the nation’s top quarterback recruit in the class of 2017, ahead of Alabama’s Tua Tagovailoa and Georgia’s Jake Fromm.

However, Mills suffered a knee injury in his senior year of high school and has been plagued by injuries since then. Stanford running back Cameron Scarlett was pleased that Mills got some significant playing time against Northwestern.

“I could just see him getting into his rhythm,” Scarlett said. “The kid hasn’t really played since his senior year in high school and he’s been through so much, and he’s just now trying to get his feet back under him.”

USC’s Slovis was less ballyhooed than Mills coming out of high school, but the Trojans’ freshman performed well enough in relief Saturday, completing 6 of 8 passes for 57 yards and an interception.

Slovis will be asked to operate the wide-open Air Raid offense that new USC offensive coordinator Graham Harrell brought with him.

“I think, talent-wise, he’s as good as I’ve ever seen,” Harrell said of Slovis last month, according to the Los Angeles Times.

Briefly: Shaw said defensive end Thomas Booker is questionable for Saturday’s game and offensive lineman Dylan Powell is doubtful.

Jake Curtis is a freelance writer.