(Photo credit: John Leyba-USA TODAY Sports)

No. 9 Southern California features the nation's highest-scoring offense but continues to have concerns on defense as it heads into a Saturday night Pac-12 game against Arizona in Los Angeles.

The Trojans (5-0, 3-0 Pac-12) are coming off back-to-back weeks in which they let two big underdogs stickaround to make things interesting in fourth quarter. USC needed a fourth-quarter touchdown to finally subdue Arizona State 42-28 on Sept. 23 in Tempe, Ariz., and held on last Saturday to win at Colorado 48-41 after holding a 27-point, third-quarter lead.

A leaky defense -- ranked 98th nationally in yards allowed (404.4 per game) -- is the reason USC has slid from a high of No. 5 in the Associated Press poll despite not losing a game.

Trojans coach Lincoln Riley said his players might have "taken their foot off the gas" against Colorado, adding that he was pleased with how his squad started this week.

"I think our team is learning a lot," he said. "This Monday practice we just had is one of the best practices we had as a team, period. I know they're excited to attack the areas we need to attack."

Arizona (3-2, 1-1) is facing a second consecutive Top 10 opponent after falling 31-24 at home to No. 7 Washington last Saturday. The Wildcats trailed by double digits much of the game but scored a late touchdown before failing to recover an onside kick with about one minute left.

The Wildcats still have uncertainty at quarterback. Jayden de Laura missed last week's game due to an ankle injury, and Noah Fifita performed well in his first start. Fifita was 27 of 39 for 232 yards with three touchdowns and one interception.

De Laura has been turnover-prone: He threw a Pac-12 high 13 interceptions last season, and he has been picked off five times in four games this season. However, coach Jedd Fisch said Monday "it would be a safe statement to make" that de Laura is the starter if healthy. The quarterback's status will be determined later in the week.

"I'm excited that if Noah is playing this coming week, he gives us a good chance to win," Fisch said.

Washington's Michael Penix Jr., a strong 2023 Heisman Trophy contender, completed 30 of 40 passes for 363 yards, albeit without a touchdown, against Arizona. Now the Wildcats face 2022 Heisman winner Caleb Williams, in the midst of another stunning season.

Williams has led USC to an average of 53.6 points per game, completing 105 of 141 passes with 21 touchdowns and one interception. His passing efficiency rating of 217.7 leads the country. Arizona, meanwhile, has not intercepted a pass this season.

Trojans running back MarShawn Lloyd is averaging 86.6 rushing yards per game and 8.3 per carry. A deep group of receivers includes Brenden Rice, who has seven TDs among his 17 receptions; and Dorian Singer, who transferred from Arizona after earning second-team All-Pac-12 honors last season. Singer has 12 catches this season, one of six Trojans with double-digit receptions.

Riley said after Tuesday's practice that he was "hopeful" to have wide receiver Zachariah Branch and safety Max Williams back for Saturday after they missed last weekend's game. Branch has a punt return and a kick return for touchdowns this season.

"Both are progressing," Riley said. "Those are two prominent players for us."

Singer is one of the three Arizona transfers on the USC roster, the others being cornerback Christian Roland-Wallace (14 tackles, one interception) and defensive lineman Kyon Barrs (four tackles).

USC has a 10-game winning streak in the series, but the matchup is often closer than the oddsmakers expect. Seven of the past 10 games have been decided by no more than one possession, and 13 of the past 16 meetings have ended with a spread of eight points or fewer.