USC quarterback Mitch Mustain knows he needs to be better this week if he hopes to overtake frontrunner Mark Sanchez before the Trojans name a starter by the end of spring practice. “I just need to study our (game plan) and make sure I understand it,” Mustain said. “I feel all right. I just have to get a lot better.” Although it is not unusual for a quarterback learning the system to experience growing pains, Mustain’s margin of error is shrinking if he hopes to win the job.