USC quarterback Matt Barkley shows a flash of anger, just for a moment

After four weeks of mild frustration, USC quarterback Matt Barkley apparently could not help himself.

An inexperienced offensive line has been unable to fuel a consistent running attack or stem a pass rush during spring practice. The receiving corps has been thinned on various days by injuries and disciplinary issues.

So on Tuesday, Barkley momentarily steamed.

The junior uncharacteristically barked at a teammate after a walk-on receiver ran the wrong route during a scrimmage drill. Nothing unusual, perhaps, for some quarterbacks, but the perpetually upbeat Barkley has rarely raised his voice in two years as the starter.

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“I’m at the point where I know what’s going on so I know my reads,” said Barkley, who is participating in his third spring practice. “When someone else messes up, it kind of throws me off.

“But it’s important to just keep your cool and realize that they’re young guys and you have to kind of coach them at the same time. I was probably the same way when I was younger too.”

Barkley isn’t the only one feeling frustration.

Coach Lane Kiffin has lamented the absence of 10 players who have sat out spring practice because of injuries and more than a dozen others who have been sidelined at various times.

Kiffin said that “a number of guys” who are practicing were “coasting.”

“There’s no one behind them,” Kiffin said. “And so they know, regardless, they can’t be demoted. Hopefully that’s something we’ll fix in the fall.”

Kiffin has praised Barkley throughout spring workouts but said the leader of the offense must strive to be “intelligent in how he’s being aggressive” when playing through frustration.

“He’s had an issue with that since he’s been here,” Kiffin said. “It’s something we think we improved on last year from Year 1 and, hopefully, we’ll improve on it in Year 3.”

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Barkley looked to be headed toward a potentially serious injury when he attempted to chase down cornerback Nickell Robey after an interception.

Defensive lineman George Uko lowered his shoulder into Barkley, who found himself momentarily sandwiched between the 6-foot-4, 300-pound Uko and 6-3 safety T.J. McDonald.

“I was just going to go by and tap him, just like I normally do,” Barkley said, laughing, “and I got a total surprise.”

Said Uko: “It was just a natural reaction because he wasn’t slowing down and I wasn’t going to let him hit me.”

Barkley was unhurt.

Quick hits

Cornerback Tony Burnett and safety Demetrius Wright sat out practice because of ankle sprains. . . . Kiffin praised linebackers Will Andrew and Marquis Simmons, who have taken advantage of opportunities afforded by the injury-related absences of projected starters Chris Galippo, Devon Kennard and Shane Horton.

gary.klein@latimes.com

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