The junior, however, practiced Sunday, offering possible help for a Trojans secondary that was exposed the last two games.
Brown, who started the opener, worked with the first-team defense during a one-hour practice that was conducted without helmets or pads.
Defensive coordinator Clancy Pendergast also spent extra time with Brown after the workout.
"We don't know if he's ready yet," Orgeron said. "It's going to very, very critical to find out."
USC gave up 351 yards passing and three touchdown passes in its 62-41 loss at
On Thursday, the Trojans surrendered 363 yards passing and four touchdown passes in a 38-31 victory over Arizona. The Wildcats went into the game averaging only 111.2 yards passing per game.
USC gave up two fourth-quarter touchdown passes, turning what appeared to be a relatively comfortable victory into a close game.
USC's defensive problems are correctable, Orgeron said.
"We let the guys get behind us a little too much," Orgeron said. "We just emphasized being in the right positions."
Safety Dion Bailey blamed himself after the Arizona game and said the Trojans' defense didn't "finish" as planned.
"I don't know what the issue was, but we didn't play up to our potential," he said.
Here come the Irish
Orgeron said the Trojans would attempt to "continue what we've built here last week," catching himself to emphasize "it's been a short time."
The Trojans will "stay with the philosophy of having fun" and playing with enthusiasm and a physical style as they attempt to "take the next step," Orgeron said.
"We realize that this is a rivalry game," he said of Notre Dame. "We have a lot of respect for our opponent, but, again, we need to take care of what we can control."
Orgeron spent much of Saturday huddling with new defensive line assistant Pete Jenkins, a 72-year-old coaching veteran who worked for the Philadelphia Eagles, Louisiana State and numerous other college programs in the South.
Orgeron said the experience "was like Christmas for me."
Jenkins' hiring was made official a few hours before the Arizona game and he was on the sideline for the victory.
Orgeron was the first coach on the field Sunday, demonstrating drills.
"He's just a plus to be here," Orgeron said of Jenkins.
All-American receiver Marqise Lee, who sat out against Arizona because of a knee sprain, attended practice and went through stretching drills with teammates, but he worked mainly with athletic trainers during the workout. Tailback Tre Madden (hamstring), tight end Xavier Grimble (shoulder) and receiver Victor Blackwell (ankle) attended practice but sat out. Linebacker Morgan Breslin (hip) participated in the walk-through portion of practice and performed conditioning drills. Receivers Darreus Rogers and De'Von Flournoy practiced after sitting out against Arizona because of injuries. Orgeron listed Lee, Breslin and Blackwell as questionable, Madden and Grimble probable. … Senior Silas Redd, who made his season debut against Arizona, is listed as a co-starter with Madden and Justin Davis on the depth chart for the Notre Dame game.