USC’s Porter Gustin to undergo MRI exam after injuring knee in practice

USC senior outside linebacker Porter Gustin was carted off the field with a knee injury during Tuesday’s practice. He is expected to undergo an MRI to determine the severity, coach Clay Helton said, adding that the exam is “extremely precautionary.”

“The knee seems stable from what the doctors said,” Helton said. “But let me wait to see the MRI before I start guessing on what it is.

“He was not in a lot of pain; not at all. It felt funny to him. If anything is off with Porter, anytime you have that fine of an athlete and something feels a little funny, obviously you want to know what it is.”

Gustin knows his body. As the “predator” of USC’s defense, he takes great pride in the sacrifices he has made to turn himself into a 6-foot-5, 260-pound quarterback hunter — including giving up eating sweets in the eighth grade.

So it comes with excruciating irony that Gustin, of all the Trojans, keeps getting sidelined with injuries.

Last season in the second game against Stanford, he injured his right big toe and tore his right biceps. He underwent toe surgery the next week and tried to play against Texas, tallying two sacks before leaving the game. Due to the toe injury lingering, he missed the rest of the season other than a brief appearance against Arizona State.

“It’s probably the most frustrating part,” Gustin said at Pac-12 media day in July. “You’re doing all of these soft tissue things to prevent injury 24-7, then all of a sudden you break a small bone like that, that you don’t even realize how much you need it playing football. It’s that much tougher, but you sit out and you learn from it. Really all you can do is pray it doesn’t happen again.”

Helton picked Gustin to represent the Trojans at media day for a reason. He’s capable of so much when he is on the field.

“It’s very tough,” Helton said. “You’re talking about one of the leaders of your football team and a great player. You catch your breath, you say a prayer for him and it not being too serious. I don’t know if there’s anybody that’s worked harder to be ready for this season on our football team.”

USC adds lineman who hit ref

Helton said USC has added transfer Bernard Schirmer, a junior college offensive lineman, to its active roster for the fall. In 2016, Schirmer, then a freshman at Mt. San Antonio College, was arrested for hitting a referee during a game. The Southern California Football Assn. suspended him for five years.

Schirmer apologized for the incident and said it was not his intention to hit the referee. According to the Ventura County Star, charges were not filed for assault because investigators were not convinced they could prove to a jury that Schirmer’s intent was to harm the referee.

A video of the incident was captured and widely circulated.

“Bernard Schirmer is a young man from Mt. SAC that we learned about in spring recruiting,” Helton said. “We knew his history with an in-game incident that he had to pay penance for. We vetted for three months this individual, talking to administrators at Mt. SAC, his coaches, his counselors and also had chance to sit down with his family over the summer. What we learned is that we were dealing with a tremendous young man … a guy that we had no question would be a great member of our Trojan family.”

Schirmer, a Long Beach native who played at Lakewood High, reportedly committed to Utah earlier this summer. His sister, Brooklyn Schirmer, will be a freshman for the USC women’s volleyball team this fall. Their uncle, Titus Tuiasosopo, played football at USC (1990-92) and their cousin, Vavae Malepeai, is a Trojans tailback.

“We listed the expectations that we expect for Bernard,” Helton said, “and we understand that he knows those expectations and will do a great job here as a Trojan.”

Schirmer will be a redshirt sophomore with three years of eligibility remaining.

Etc.

Helton said USC hired former Dallas Cowboys coach Dave Campo as a consultant. Campo is a mentor of USC defensive coordinator Clancy Pendergast. … The Trojans will have their first practice in full pads Wednesday and will have a short, live scrimmage to end practice. … Senior linebacker Cameron Smith missed his second practice in a row with a tight hamstring. … Senior right tackle Chuma Edoga left practice early with hip soreness. … Redshirt freshman cornerback Je’Quari Godfrey was also out with hip soreness. … Sophomore linebacker Levi Jones is sitting because of swelling in his chest. Helton said it is “not coronary” and is precautionary.

brady.mccollough@latimes.com

Twitter: @BradyMcCollough