The setup: Senior backup quarterback Mitch Mustain made his long-awaited first start in a Trojan uniform after sophomore quarterback Matt Barkley went down with a high-ankle sprain in last week’s loss to Oregon State. Mustain led the Trojans against a struggling Notre Dame team in a game that had little hype and fanfare as for the first time since 2001, both teams came into the game unranked.The story: It was a fairytale ending for Mustain. The Trojans had the game-winning touchdown in the final minute in their grasp.Then everything slipped away.Tim Tran | Daily TrojanIn a twist of emotion found only in movies, the Trojans went from utter jubilation to shock and disappointment as senior receiver Ronald Johnson, who is usually sure-handed, dropped a 53-yard touchdown that would’ve put USC up 23-20 with just 1:17 remaining in USC’s last game in the Coliseum this year.“I was shocked [Johnson dropped the pass],” USC coach Lane Kiffin said. “It’s something that we’ve worked on with that coverage. It’s just a shame.”The blame can’t be put solely on Johnson though. USC still had multiple chances after that drop with a 14-yard completion to redshirt junior tight end Rhett Ellison on fourth-and-seven and a 16-yard completion to freshman receiver Robert Woods on the next play. However, Mustain would throw an interception to end the drive.USC receivers were dropping passes all night long as they didn’t make life any easier for the first-time Trojan starter.Cameron attributed lack of focus to the multitude of drops.“It was concentration,” senior tight end Jordan Cameron said. “You got to make plays and be focused. I think a lot of guys took their eyes off the ball focusing on the yards after the catch but you need to focus when it comes to you. You feel terrible when you drop the ball. It happens to the best of us, you gotta have a short memory.”Defense rules: In a dramatic turn from where this team was at the beginning of the season, the USC defense was the unit keeping the team in the game with four takeaways that led to all of USC’s points.On top of only allowing 15 first downs, the second fewest the USC defense has allowed this year, the defense picked off Notre Dame quarterback Tommy Rees three times and advanced a fumble to the Fighting Irish’s two-yard line that led to the only touchdown from the Trojans.“It was a surprising game from the aspect that prior to the last play of the game we were plus-four in turnover margin,” Kiffin said. “You think that you’d win that game but obviously we got to make more points off the turnovers.”However, two-minute defense was the one thing that was reminded everyone this was still the same unit as the one that took the field against Washington and Stanford earlier this year. All three of Notre Dame’s touchdowns came with less than 2:39 left in each half.“I’m not sure [why we can’t stop teams in the two-minute drill],” sophomore linebacker Devon Kennard said. “Today was a little different in that they were getting big runs on us but usually it’s big passes. We got to play more consistent in the second and fourth quarters.”Injured Trojans: While Mustain made his first start of his Trojan career, Barkley suited up and participated in the captains’ coin toss at the start of the game but wore a baseball cap on the sidelines during the entire game. He said he knew sometime before the game that he wouldn’t be able to play but said that it was tough watching it from the sidelines and is anxious to get back on the field next week.“I’m going to play this week. There’s no doubt about that,” Barkley said. “Whatever needs to happen, I’m going to make it happen.”Barkley’s injury occurred last week, but one important Trojan got injured this week. Sophomore safety T.J. McDonald, who leads the team in tackles, left the game after making tackle on the first play of the second series of the game. McDonald separated his shoulder and did not return.“I came and it was a big hit on the sidelines. I got up and it didn’t feel right. I knew something was wrong,” McDonald said.Walk-on sophomore Tony Burnett took McDonald’s place and recorded 10 tackles, second-highest on the team.“He did a great job coming in for me,” McDonald said. “I was helping him on the sidelines and to be put in my position in a rivalry game, that’s a big job and for him to come in a do that, that’s great.”The Trojans are very thin in a secondary now with sophomore safety Jawanza Starling, redshirt freshman cornerback Torin Harris and junior cornerback T.J. Bryant not playing because of injury — that was already extremely young and inexperienced with it’s starters at the beginning of the year.Senior Day blues: For Mustain, Johnson and a few other key Trojans on the field, this was the last game they played in the Coliseum. To go out in a loss to Notre Dame with the game decided in the final minute wasn’t the way many of these seniors expected to go out.“It’s extremely disappointing. As disappointing as it gets,” senior linebacker Malcolm Smith said.Lost in the crowd: USC senior kicker Joe Houston made all three of his field goal attempts after completing only seven of 13 attempts on the season coming into this game.Houston converted on 45-, 23- and 37-yard attempts, the last one he kicked in driving rain that put the Trojans up 16-13 with 6:25 left in the fourth quarter. The 45-yard field goal was his career long, however, all that was lost in the chaotic ending to the game.