USC secondary in need of a turnaround

USC cornerback Nickell Robey doesn’t have one. Neither does T.J. McDonald nor fellow safety Jawanza Starling.

Torin Harris is the lone Trojans player with an interception this season. And the cornerback is expected to be sidelined Saturday afternoon at the Coliseum against Arizona because of a shoulder injury.

So it’s probably not the best time for turnover-starved USC (3-1) to face one of college football’s top quarterbacks.

But Trojans defensive backs can’t wait to go up against Arizona’s Nick Foles. “This is going to be the most important week for the secondary,” Robey said.

It certainly will be a test.

Foles has completed 71% of his passes and has thrown for 10 touchdowns. The 6-foot-5 senior has not had any of his last 200 passes intercepted, dating to last season.

Foles’ feat is even more impressive because Arizona — 1-3, but each of its losses to a top-10 team — has played from behind in three consecutive double-digit defeats, usually a contributing factor to turnovers.

“He’s on fire,” said Monte Kiffin, USC’s assistant head coach for defense.

USC’s secondary has not been burned consistently for long gains — a departure from 2010 — but neither has it shined.

“Up and down,” Coach Lane Kiffin said when asked to assess the unit.

Last week’s 43-22 loss at Arizona State was an example. The Trojans kept Sun Devils quarterback Brock Osweiler from completing long pass plays, but the Trojans were exposed in other ways.

Starling had the most glaring miscue. He had a clean shot at Sun Devils running back Cameron Marshall on the opening possession, but Starling failed to tackle or knock him out of bounds, allowing Marshall to complete a 70-yard touchdown run.

Robey was flagged for two pass-interference penalties, one that was declined because it occurred on a touchdown and another that kept alive a scoring drive.

McDonald was called for three personal fouls, resulting in 45 yards of penalty yardage and automatic first downs. Two of the penalties helped keep alive scoring drives.

In assessing the secondary, Lane Kiffin cited the distraction and break in continuity that resulted from assistant Willie Mack Garza’s departure. Garza, also a member of Kiffin’s 2009 Tennessee Volunteers staff, resigned on the eve of the Trojans’ opener against Minnesota, reportedly after a former talent scout told NCAA investigators that Garza had reimbursed him for a prospect’s travel on an unofficial visit to Knoxville.

Graduate assistant Sammy Knight now oversees the secondary.

“We need to tackle better,” said Knight, a former USC and NFL safety, “and create turnovers.”

The secondary is not the only unit struggling to make momentum-turning plays.

The Trojans rank 113th among 120 major college teams in turnover margin, having come up with only Harris’ interception and two fumbles while giving away the ball nine times.

The defensive line has not created consistent pressure and linebackers have missed opportunities for interceptions.

Kiffin joked this week about having offered ice cream as a reward for turnovers, to no avail.

Recapturing what was a hallmark of Pete Carroll-coached teams in the mid-2000s has been a struggle.

“We’re doing the same things that we did the last time we were here, when we were the best in the world at turnover margin,” Kiffin said. “There’s nothing different as far as emphasis and practice. It will turn around.”

The Trojans should have plenty of opportunities against Arizona.

The Wildcats haven’t been able to run the ball, and Foles threw 57 times in last week’s loss to No. 9 Oregon.

“We’ve got to make plays, disguise and get him out of his rhythm,” Starling said.

Robey, who had a team-best four interceptions last season, hopes to get his first against the Wildcats.

“We have to have some turnovers,” he said, “some way or another.”

gary.klein@latimes.com

twitter.com/latimesklein