LOS ANGELES — USC might be without one of its top pass rushers early in the season.

Outside linebacker Porter Gustin has suffered a torn meniscus in his left knee and will undergo surgery Thursday, a procedure that could sideline him for at least the Trojans’ season opener on Sept. 1 against UNLV.

The timetable for Gustin’s return is expected to be 2-5 weeks, Coach Clay Helton said, depending on his recovery.

After beginning the season against UNLV, the Trojans face road tests at Stanford and Texas the following weeks.

The meniscus tear was diagnosed late Tuesday following an MRI. In practice hours earlier, Gustin had injured his knee on a non-contact play in an 11-on-11 drill and was carted off the field. One source described it as “slightly torn.” Helton reiterated the tear was “minor” and that there was no ligament damage.

Gustin, a senior, watched the Trojans’ entire two-hour practice on Wednesday afternoon from the sideline at Howard Jones Field. He wore a sleeve over his left knee and walked without an obvious limp.

When he spoke with reporters after practice and a group of nearby boosters, Helton sounded upbeat about Gustin’s prognosis and remarked, “the good Lord is watching after the Trojans.” He was uncertain, though, if Gustin would be ready for the opener.

“You never know how people heal,” Helton said. “I always lean on two things. Is he medically cleared? And how does the player feel? If he’s a 100 percent, he’ll go. I know Porter. He’ll give it everything he has to be back as soon as possible.”

Toe and bicep injuries cost Gustin most of last season, sidelining him for 10 games. He twice attempted to return early from the injuries. Three days after screws were implanted in his big toe to repair a fracture in mid-September, he played in a double-overtime win over Texas, recording a pair of sacks, but did not return for the second half after the screws grew loose. He appeared in only one of the final 11 games, a brief return in a win at Arizona State on Halloween weekend.

During spring practice, Gustin said he had no regrets about returning against the Longhorns.

“It was a really close game,” Gustin said, “I was able to come up with a couple big plays at the end of the (first) half, maybe even preventing them from scoring at the last drive of the half. In my head, I like to think I was a factor in that win.”

The absence of Gustin for a significant length of time would serve as a significant blow to a veteran defense that is expected to be the Trojans’ stronger unit due to heavy roster turnover on offense. The pass-rushing linebacker had three sacks in the first three games last season and had 5.5 sacks during his sophomore season in 2016.

Without Gustin, the Trojans reconfigured their first-team defense in practice. Defensive lineman Christian Rector took over Gustin’s role as the “predator” linebacker, a hybrid position that lines up near the line of scrimmage. Rector assumed the role for several games last season when Gustin was injured. Freshman Kana’i Mauga, who had enrolled early for spring practice, took over as the opposite outside linebacker Wednesday.

“He’s way ahead of the game for a true freshman,” Helton said.

MORE SNAPPING ISSUES

Center Toa Lobendahn was twice pulled from 11-on-11 drills after botched snaps, an issue that has arisen in recent days of training camp.

Redshirt freshman Brett Neilon replaced Lobendahn following both snaps, as Helton sounded frustrated.

Last season, Lobendahn started for the Trojans at left tackle, but since moved to center, where the redshirt senior made three starts in 2016 and 2015.

Asked if the Trojans would consider moving Lobendahn elsewhere on the offensive line if snapping issues continued, Helton said, “We’ll get it fixed. The beauty of Toa is he’s started in three different positions. But I have no question. He gets a little bit better every day.”

CARR IN GEAR

Running back Stephen Carr provided a share of highlights Wednesday and looked healthy in his first full-padded practice since returning from offseason back surgery. In the first 11-on-11 period, Carr burst for a 25-yard touchdown and outran safety C.J. Pollard over final 10 yards.

“He did not look tentative running between the tackles,” Helton said.

Carr also caught a pair of touchdowns from quarterback Jack Sears in red-zone situations.

QUICK HITS

Sophomore kicker Chase McGrath has not kicked in recent practices due to a groin injury. Helton described it as precautionary and has allowed for Michael Brown, returning from a torn ACL, to see extended work. … Injured players who sat out Wednesday’s practice included Chuma Edoga (hip soreness), who was injured Tuesday, as well as linebacker Cameron Smith (hamstring) and safety Ykili Ross (hamstring), who have been out since Saturday. … Linebacker Levi Jones (chest) returned to practice. … Newly added junior college transfer offensive lineman Bernard Schirmer joined the Trojans at practice for the first time Wednesday and went through individual drills.