Redshirt freshman Jesse Scroggins, one of three contenders to backup Barkley, had surgery on his throwing hand, catching USC head coach Lane Kiffin off guard.
"It was a surprise," Kiffin said. "We obviously did it quick for obvious reasons. I did not think that it was going to be that severe."
Scroggins injured his right thumb Monday during the Trojans' first scrimmage and saw a specialist Wednesday.
The competition between those two remains too close to call, the loser all but guaranteed to redshirt.
"If it was clear cut, we would make that decision (right now)," Kiffin said.
Both participated in spring practice after enrolling in January. While Wittek struggled with the transition from Santa Ana (Calif.) Mater Dei, Kessler, the Bakersfield (Calif.) Centennial product, performed well enough to emerge as a legitimate challenger to Scroggins.
Through the first week of camp, Kessler has continued his strong play, dominating in the Coliseum by completing 15 of 18 passes for 187 yards, three touchdowns and one interception, but Wittek has also performed well of late.
"It's no different than before," Kiffin said. "Those guys will battle it out. They are both doing great. If you look at the last two days, those guys have been over 60 percent and well up there into the 80 percent at times.
"They don't look like freshmen, but then again this isn't the real thing either."
Scroggins isn't the only player out, as physical practices with live tackling are starting to take their toll. Linebacker Dion Bailey suffered a foot injury, joining wide receiver Robert Woods (ankle), safety T.J. McDonald (ankle), defensive tackle Christian Tupou (knee) and linebacker Chris Galippo (shoulder) on the sidelines.
However, those players are resting minor or nagging ailments and should return soon.
Kiffin praised Butler for his toughness, part of a remarkable turnaround for the redshirt junior after almost leaving the team in the spring.
"I don't know if that would have happened a year ago, so I was proud of him," Kiffin said.
-Sophomore wide receiver Markeith Ambles practiced for the first time this fall, surprisingly working with the defensive backs in individual drills.
Ambles had been held out to focus on summer school and would not practice until he was academically eligible, Kiffin said Wednesday.
"He's done with school, so there was nothing left for him to stay in," Kiffin said. "We thought it was worthwhile to have him out here running around and see him running around so that if he is eligible we can move forward."