USC coach Pete Carroll made official after practice Thursday what had been expected since the beginning of the week, when Booty was unable to throw because of the soreness and swelling in the broken middle finger on his throwing hand.
"We're counting on him to play a heck of a game," Carroll said of Sanchez, a 6-foot-3, 225-pound redshirt sophomore. "He had a great week. He's been around this offense for years. I know how excited he is -- I've got to calm him down.
"He's ready for this. He's waited for this. He's been in games," Carroll said.
Sanchez hasn't started a game since 2004, when he was a senior at nearby Mission Viejo High. He took first-team snaps Thursday for the fourth straight day.
Sanchez has completed 2-of-3 passes for 23 yards with one interception this season and is 5-of-10 for 86 yards and one touchdown with two interceptions in his career.
"I'm excited about the opportunity," Sanchez said. "I'm approaching it the same way I have all week, with focus and preparation. The coaches have prepared me well for this."
Booty, a fifth-year senior, was hurt in the second quarter of last Saturday's shocking 24-23 loss to Stanford. He played the rest of the game and passed for 364 yards but threw four interceptions after being injured when his hand hit a helmet.
"J.B. will be available to us," Carroll said. "He can throw the football a little bit. We'd love for him to not have to play."
Booty said his finger swelled up again after he threw about 50 passes on the side Tuesday. He has passed for 1,239 yards and 12 touchdowns with eight interceptions in five games this season after throwing for 3,349 yards and 29 TDs with nine interceptions in 13 games last year.
USC (4-1, 2-1 Pac-10) enters with a five-game winning streak against Arizona (2-4, 1-2) since the Wildcats won a 31-15 decision in 2000 -- the year before Carroll became the Trojans' coach.