Sanchez is getting the start as regular starter John David Booty is still recovering from a broken right middle finger, coach Pete Carroll said. Booty, who threw in practice Tuesday and Wednesday, did not throw on Thursday.
"He threw the ball well, but there was a few that got away from him and he can't tell when that was happening," Carroll said. "That amount of uncertainty was enough to give him another week. Hopefully, Mark can make it through the game and we can get a win and we'll go onto next week and figure it out then."
But Carroll also said Booty could play in relief of Sanchez if needed.
Booty had said Wednesday he felt like he was getting closer to returning, but was leaving the decision up to his coach.
Sanchez threw for four touchdowns in USC's 38-0 win over Notre Dame last week. He passed for 130 yards and a touchdown but was intercepted twice in a 20-13 victory over Arizona.
His father, an Orange County firefighter, has been battling the wildfires that forced the evacuation of nearly one million people in greater San Diego this week.
"I've been getting text messages from my dad and his unit, so I know he's doing all right," Sanchez told the Orange County Register on Wednesday. "I think about it during the day. I think about it when I see the news to see if anything is going on that I need to be aware or worried about."