USC cornerback Iman Marshall is injured and Notre Dame picks on his replacement

Iman Marshall sat on the trainer’s table in the first quarter of USC’s loss to Notre Dame with his head down and welcomed a procession of teammates and coaches.

Marshall’s left knee was propped up and wrapped in ice. When coach Clay Helton came over with about five minutes left in the first quarter, he held the back of Marshall’s head and said a few words. The cornerback’s night was over.

Marshall appeared to sustain the injury early in the first quarter after two series. The exact injury and its severity was not immediately known.

USC used Isaiah Langley in Marshall’s place. Notre Dame tested him immediately. Equanimeous St. Brown had Langley beaten by two steps, but quarterback Brandon Wimbush overthrew him. Three plays later, Langley had Kevin Stepherson covered but Wimbush threw a back-shoulder pass and Stepherson made an acrobatic catch for a touchdown.

Marshall was only the latest in a rash of injuries for USC. Eleven starters other have missed a game because of injury, the latest being defensive tackle Josh Fatu, who missed Saturday’s game after sustaining a concussion in a car accident this week. Receiver Jalen Greene, who was a starter but isn’t currently, also missed a game because of a concussion.

Darnold banged up

Late in the third quarter, Sam Darnold was trying to slip through another fierce Notre Dame pass rush and had slipped past the last rusher when he was caught by his left ankle. It wrenched unnaturally.

Darnold came up limping and was pulled for one play. The injury did not appear severe — he did not receive medical attention on the sideline. After a timeout and a break for the start of the fourth quarter, Darnold returned for one play, a fourth-down try that was unsuccessful.

But the next series, Matt Fink replaced him and Darnold did not appear for the rest of the game.

Rule stings USC

USC was victimized by its own field position in the second quarter on a rule that might have cost USC a score.

On a third-and-four from the six-yard line, Darnold was sacked, but Notre Dame was flagged for holding. USC appeared to anticipate that it would be a first down — the television broadcast and the official stats feed both indicated it would be a first down. But the down marker on the field still read third down.

Helton called timeout and talked animatedly with the referees.

According to the NCAA rulebook, the ruling was the right one, if rarely occurring. Usually, a holding penalty is worth five yards — enough to give USC a first down. But USC was close enough to the goal line that the penalty gave them half the distance to the goal.

The penalty is an automatic first down only if the pass crosses the line of scrimmage. Since Darnold was sacked, there was no automatic first down.

USC lost a yard on third down, attempted a field goal and missed.

Returner shuffle

USC made a switch at punt returner, opting for Jack Jones in place of Ajene Harris, who had performed that duty in USC’s first seven games.

Harris tended to let the ball bounce without fielding it. He had just six returns for a total of 26 yards this season.

Jones fielded and returned the first two punts USC saw against Notre Dame for positive yardage — though the second one was negated by a penalty.

But on his third punt, disaster hit. Jones tried to fair catch a punt inside USC’s 10-yard line. He dropped it. Notre Dame recovered and scored three plays later.

Jones finished with 48 yards on four returns.

Turnovers a problem again

Before Saturday, USC’s defense had given up only 10 points off 14 turnovers this season. (The offense had given up another two return scores off an interception and a fumble.)

Notre Dame, meanwhile, had scored on 11 of the 14 turnovers its defense had forced.

Something had to give. It was USC. The Trojans had three turnovers. They gave up points on all of them.

zach.helfand@latimes.com

Follow Zach Helfand on Twitter @zhelfand