Running back Marc Tyler will be out indefinitely after pulling his right hamstring on Tuesday.
"It's a nagging injury and it takes a lot of rehab," said Tyler, who led the team with 930 yards rushing and nine touchdowns last season.
With the senior out, it leaves USC with just three scholarship running backs available.
Mesa in Charge
Justin Mesa has been a constant presence at USC for years, doing everything from checking in visitors at Howard Jones Field to signaling in plays at the Coliseum.
Now the 28-year old has a new role, overseeing Trojans tight ends in his second stint as a graduate assistant.
"It's awesome," Mesa said. "Coach Kiffin, (former wide receivers coach John Morton) Johnny Mo, Coach Carroll, they just allowed me to stick around and learn. Eventually, I worked myself into a spot where I had this opportunity. I got into in because I wanted to coach and this is what I wanted to do with my life."
Mesa takes over for Keary Colbert, who left to return to the NFL, and inherits a group lacking experience.
Sophomore Christian Thomas and redshirt freshmen Xavier Grimble and Randall Telfer were all touted recruits, but none caught a pass last season. Thomas bounced around between tight end, defensive end and even fullback.
"None of them really got a lot of time to spend at the position," Mesa said.
"We're short numbers here, so it's awesome for them because they are getting tons of experience. Just hearing the terminology, just hearing me be their tight end coach."
Senior incumbent Rhett Ellison, who has 31 career receptions and four touchdowns, will serve as both stabilizing force and mentor.
"I've empowered Rhett to take the leadership role," Mesa said. "I hope they lean on him outside of our room or when they're all together, maybe eating lunch or hanging out in the dorms. Rhett has the power to go ahead and talk to these guys.
"When Rhett speaks, they listen."
Ellison could also end up at fullback, as the team has just one true scholarship fullback on the roster in Soma Vainuku. He did some work as an H-back, motioning in and out of the backfield last season.
"Rhett is pretty knowledgeable about that," Mesa said. "He's worked with Coach Pola in the offseason about getting better at that fullback stuff, the run blocking and some passing routes."
If Vainuku does not make an immediate impact, USC could lean on more two tight end sets, three if Ellison or Thomas line up at fullback. Mesa has everyone learning how to play both spots.
The force-feeding seems to be working, as Grimble had an impressive one-handed catch reminiscent of Dominique Byrd's at Oregon State.
"I was pumped," Mesa said.
It was also among the few offensive highlights on a day filled with drops and turnovers.
"We obviously have a lot of work to do on offense," coach Lane Kiffin said.
Still, he was pleased with how the defense capitalized on those mistakes.
"It was a practice that was pretty much defensive dominated," Kiffin said. "They limited big plays again, which obviously has been a gigantic emphasis over the offseason, especially in the passing game."
-Kiffin gave early enrollees Andre Heidari and long snapper Peter McBride solid marks.
"He's got a strong leg," Kiffin said of Heidari.
"The snapper and the kicker, for guys that were supposed to be seniors in high school, are really at the top of their game ability-wise, from the speed of the snap to the kick and the leg. It's great to have them here already."
-Kiffin indicated morning practices would continue in the fall, citing the benefits it has both academically and on the field.
"I think it's very beneficial to our kids," he said. "They have great energy. It's a lot easier I think on their bodies to have good practices and start at 7 in the morning as opposed to going through the whole day of class and lunch and getting out here at 4 in the afternoon."
-Jacksonville Jaguars coach and former USC linebacker Jack Del Rio attended practice with his son Luke, a high school quarterback.