CALL FOR BACKUP

There were times Mark Sanchez thought his opportunity at USC might never arrive.

The former Mission Viejo High quarterback sat behind Matt Leinart and John David Booty in 2005, backed up Booty last season and seemed destined for another season of mop-up duty as part of the run-up for a battle with Arkansas transfer Mitch Mustain in the spring.

But Coach Pete Carroll officially pushed up Sanchez’s schedule Thursday when he announced that the third-year sophomore would start Saturday’s Pacific 10 Conference game against Arizona at the Coliseum.

Booty, who struggled against Washington and suffered a broken middle finger on his throwing hand in last week’s stunning loss to Stanford, will sit out so he can heal, though he will be available if needed as an emergency backup.

Carroll said Sanchez was prepared for the opportunity.

“We’re counting on him to play a heck of a football game,” Carroll said. “He had a very, very good week and he’s been waiting for this for years. I just don’t want him to try and think he has to do everything the first time he gets out there.”

Carroll went into full passer-protection mode on Thursday, cutting off access to both quarterbacks and instructing the sports information staff to issue prepared statements.

“I’m excited about the opportunity. I’m approaching it the same way I have all week, with focus and preparation,” Sanchez said. “The coaches have prepared me well for this. I’ve watched film and studied the plays, and now I’m ready.”

Said Booty: “I’ll be right there next to Mark the whole way to help him along. He’ll do a great job. He’s prepared himself well, he’s waited his turn and now he’s ready to play.”

The decision to go with Sanchez marks a first in the Carroll era.

Heisman Trophy winners Carson Palmer and Leinart never missed a start because of injury.

Sanchez, however, is not completely injury free. On Wednesday he jammed his left thumb, and he wore a heavy bandage on his non-throwing hand during practice Thursday.

Bob Johnson, the father of former USC quarterback Rob Johnson, tutored Palmer as a private instructor and coached Sanchez at Mission Viejo. He is confident that Sanchez will rise to the occasion as the Trojans try to rebound from one of the biggest upsets in college football history.

“He’s ready, he’s been ready and he’s not going to panic out there,” Johnson said. “He’s a very confident kid who’s waited his turn and I think he’s going to run with it.”

The 6-foot-3, 225-pound Sanchez is more mobile than Booty and possesses what Carroll described as “a huge arm.”

“He can throw the ball a mile and he loves to air it out,” Carroll said. “He always has been a guy that’s looking for his chance to make a big play and look down field, so that’s always been Mark’s kind of manner and we’ve always loved that about him.”

The challenge for Sanchez, however, will be playing within the offense and not forcing throws. He has completed five of 10 passes for 86 yards in eight games over two seasons, with one touchdown and two interceptions.

“We just don’t want to get him over-hyped or worked up, and we want him to make good decisions,” Carroll said.

Earlier this week, Sanchez said he had gone through the “metamorphosis” experienced by many quarterbacks when they transition to college football.

“In high school I got to throw the ball all over the yard and it was great, so more than anything I just need to manage the offense here, check the ball down if I have to and be smart,” he said.

Sanchez helped Mission Viejo win the 2004 Southern Section Division II championship, passing for 24 touchdowns with only four interceptions. During his prep career, he also played against Northern California power Concord De La Salle and in several postseason all-star games.

“He played in huge games, giant games against a lot of Division I players, but college ball is a lot bigger than that,” Johnson said.

Sanchez redshirted as planned in 2005 while Leinart played his final season with Booty as the backup.

A competition for the starting job in 2006 went by the wayside when Booty underwent back surgery after the first day of spring practice. Sanchez ran the first-team offense while Booty recuperated but coaches made it clear that Booty would be regarded as the No. 1 quarterback when training camp began.

Sanchez, however, had a more serious issue to deal with in April 2006 when he was arrested on suspicion of sexual assault. After a two-month investigation, no charges were filed.

“I’m so far past that now, it’s just a matter of learning from a tough experience,” he said this week.

Sanchez played in six games last season, completing three of seven passes for 63 yards. Booty’s outstanding performance against Michigan in the Rose Bowl eliminated any chance of a quarterback battle in the spring and Sanchez entered this season knowing he would probably sit again.

After suffering a broken right thumb during a training camp scrimmage, he was sidelined for three weeks and was unavailable for the season opener against Idaho. He played late in games against Nebraska and Washington State and settled into his role as a backup.

But two of Booty’s passes were intercepted in a 27-24 victory over Washington on Sept. 29, then four more last week in the 24-23 loss to Stanford after he broke his finger in the first half.

Now, for the first time in three years, Sanchez is preparing to command the huddle as a starter.

His father, Nick, said he would have some simple advice for the youngest of his three sons.

“I’ll probably tell him, ‘Everything is not going to go perfect, this isn’t a storybook,’ ” Nick Sanchez said. “You’re going to make mistakes but you have to find a way to turn it around and make good things happen for your team.

“Just use the leadership qualities you exude and just go out and do what you do.”

gary.klein@latimes.com