Sanchez’s back forces quarterback shuffle

USC shuffled its quarterbacks Tuesday because Mark Sanchez is suffering from lingering back soreness.

Sanchez, a redshirt freshman who is No. 2 on the depth chart behind John David Booty, was held out of practice and said he was day to day.

Junior Michael McDonald took repetitions with the second-team offense, and freshman Garrett Green, who switched from quarterback to safety during training camp, moved back to offense and ran the scout team.

Coach Pete Carroll described the shuffle as “a little emergency status move,” and said McDonald would be the No. 2 quarterback Saturday at Oregon State if Sanchez were unable to play.

Sanchez said he might have injured his back lifting weights but was not certain.

“It’s not serious,” he said. “It’s no big deal.”

Carroll said Sanchez had undergone “considerable testing” to determine the cause of the discomfort. Doctors ruled out a disk problem, Carroll said.

Booty had surgery in March to correct a disk condition that he aggravated on the first day of spring practice. Booty said he counseled Sanchez about managing discomfort.

“I told him to get that thing right,” Booty said. “If you can stop it now before it gets to where it may become something really serious, do it.”

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Tailback Chauncey Washington’s performance against Arizona State earned him a solo spot atop the depth chart for the first time this season.

But Washington, who returned after sitting out Monday’s practice to take a midterm exam, said he needed to lose about five pounds.

“I’ll be faster,” he said. “I’m separating, but I’d like to separate more.”

Washington said he ate “some of my grandma’s food” during the Trojans’ three-day break last weekend and went from 220 pounds to 225.

gary.klein@latimes.com