Defense led way in second half

Looking back at No. 13 USC’s 20-13 victory over Arizona.

Rewind

As expected: With an injury-depleted offensive line even before the game began, the Trojans mostly struggled to run the ball effectively.

Mark Sanchez appeared to have a lot of nervous energy at the start and made some ill-advised throws that resulted in two interceptions, but the sophomore was poised when he needed to be and showed the skill-set that made him one of the nation’s most sought-after recruits in 2005.

The defense kept Arizona’s spread offense in check and shut it down in the second half.

Unexpected: Coach Pete Carroll maintained throughout the week that senior tailback Hershel Dennis would back up starter Chauncey Washington, but freshman Joe McKnight got the call.

A conservative game plan was no surprise, but not allowing Sanchez to attempt a pass on three third-down situations in the first half was. “We wanted to make sure not to expose Mark to a lot of difficult situations right off the bat,” Carroll said Sunday.

Sanchez, however, was asked to make a deep throw with the ball on the Trojans’ two-yard line in the second quarter, which resulted in an interception. He also threw a first-down pass with just over a minute left in the first half, which also was intercepted but, luckily for USC, was fumbled back to the Trojans.

Despite the victory, the Trojans fell from 10th to 13th in the Associated Press media poll and from seventh to ninth in the coaches’ and Harris polls.

Stepped up: McKnight sparked the Trojans to victory with his 45-yard punt return and a 59-yard run from scrimmage. Sanchez showed his mobility, made plays off the run and delivered the game-winning touchdown pass in the clutch.

Redshirt freshman Butch Lewis replaced All-American Sam Baker at left tackle after Baker suffered a hamstring injury. Senior Alatini Malu filled in for Zack Heberer (shoulder) despite a hamstring injury. Third-year sophomore Luthur Brown made six tackles after middle linebacker Rey Maualuga left because of a hip injury.

Quarterback John David Booty did not play because of a broken middle finger, but Sanchez repeatedly credited the senior for helping and encouraging him throughout the game.

Stepped back: Mounting injuries caused the offensive line to play in near survival mode rather than attacking the defense.

Carroll is hoping that guard Chilo Rachal can return soon from a knee injury to bolster a group that will need to be stronger for a second-half run that includes games at Oregon, California and Arizona State.

Carroll on Sanchez’s performance: “There’s no question that helps Mark get better prepared for his competition [with Mitch Mustain] for the spot next year. . . . In the worst of times there’s some good things that occur.”

Carroll said the focus on developing a young receiving corps and the mounting injuries have the Trojans “kind of in flux” at the season’s mid-point.

Asked if he regretted not turning McKnight loose against Stanford, Carroll said, “In retrospect, I wish we had done it the first week, just left him in and played. We thought it would be better to spot play him early. As it turns out, it was not.”

Carroll said the Trojans’ defense “has been just rock solid the whole time. We might have had a couple plays we regret -- we certainly do in the Stanford game -- but other than that it’s been a terrific season for the defense.”

Injury report: Booty is expected to try and test his finger early in the week, setting up a potential sticky situation for Carroll if the senior shows he is fine.

Carroll said he did not know if Baker or Maualuga (hip) would be able to play against Notre Dame and was unsure about the severity of Kevin Ellison’s nose injury.

Running back Stafon Johnson (foot) will try to return this week and Rachal and linebacker Brian Cushing (ankle) also may test their fitness.

Looking ahead to the game vs. Notre Dame (Saturday, Notre Dame Stadium, South Bend, Ind., 12:30 p.m. PDT, Channel 4).

Fast forward

First look: Notre Dame, coming off a 27-14 loss to Boston College, is still struggling to generate offense.

Topic of the week: Who will start at quarterback.

Carroll said that Sanchez would begin the week as the starter, but what if Booty throws passes Tuesday and appears sound?

“There’s no point in me speculating and we’ll see what happens,” Carroll said. “I don’t know. I don’t have to do that so I’m not going to. I’m not sure the doctors will let him throw on Tuesday.”

Evan Sharpley replaced Jimmy Clausen at quarterback for Notre Dame against Boston College and passed for a touchdown.

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gary.klein@latimes.com

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(BEGIN TEXT OF INFOBOX)

Season log (5-1)

Sept 1: Idaho (1-6); Won, 38-10.

Sept. 15: at then-No. 14 Nebraska (4-3); Won 49-31.

Sept. 22: Washington State (2-5, 0-4 in Pac-10); Won, 47-14.

Sept. 29: Washington (2-4, 0-3); Won, 27-24.

Oct. 6: Stanford (2-4, 1-3); Lost, 24-23.

Oct. 13: Arizona (2-5, 1-3); Won, 20-13; Wildcats might one day look back and legitimately claim they were there when Joe McKnight burst onto the national scene.

Saturday: at Notre Dame (1-6); The Fighting Irish can boast that they own the Pac-10, and Los Angeles, if their second win comes against USC.

Oct. 27: at Oregon (5-1, 2-1)

Nov. 3: Oregon State (4-3, 2-2)

Nov. 10: at California (5-1, 2-1)

Nov. 22: at Arizona State (7-0, 4-0)

Dec. 1: UCLA (4-2, 3-0)