Notre Dame’s Brian Kelly waves as he walks off the field at Los Angeles Memorial Coliseum after defeating the USC Trojans on Saturday. (AP)

LOS ANGELES — Jack Swarbrick stood against a wall in the tunnel at the LA Coliseum on Saturday night, his mind replaying where the Notre Dame football program was the last time it played USC here.

“We left this place flat on our backs,” the Fighting Irish athletic director said.

This time, Notre Dame leaves this place undefeated and owning the first locked-up bid to the 2018 College Football Playoff. The Irish are 12-0, having completed a perfect regular season with an occasionally tense, 24-17 comeback victory over USC. How they got here from the 4-8 2016 season, which ended with a beating at the hands of the Trojans, is little short of amazing.

“It was a really critical time for the program,” Swarbrick said.

Of critical importance in that critical time: Swarbrick never considered firing head coach Brian Kelly. He said as much midway through that ’16 season, and he stuck by him even as the losses mounted. But there would be major changes to come.

“A lot of things had to change, including me,” Kelly said. ” … We didn’t reinvent the wheel here. Behind the scenes, we made some tweaks.”

Truth is, those “tweaks” were titanic. There were new coordinators across the board: offense, defense, special teams, strength coach. A re-examination of practice structure. Changes to offseason programs. Even the head coach’s attitude and occasionally volcanic demeanor were open to re-evaluation.

Guess what? Change can be good. Kelly didn’t just survive the bad times, he’s thrived.

Since that low point in LA on Nov. 26, 2016, Notre Dame has gone 22-3. But even from the bounce-back 10-3 record of ’17 to the breakthrough 12-0 of ’18, there were additional changes that had to be made.

Star defensive coordinator Mike Elko was swiped by Texas A&M. And the guy who was supposed to be the starting quarterback, Brandon Wimbush, needed to be replaced three games in. Saturday night, former backup Ian Book again got the job done at QB and first-year coordinator Clark Lea’s halftime defensive adjustments shut down the Trojans.

Having navigated a turbulent cycle and emerged at the top of his game, it’s time for a new appreciation of Kelly. In a lot of programs, when the wheels come off they are very hard to put back on with the same coach. They’ve done it at Notre Dame, and now Kelly has two undefeated regular seasons in South Bend. (Plus a third at Cincinnati, as he reminded the media postgame.)

The last Notre Dame coach to go undefeated twice was the great Ara Parseghian, in 1966 and 1973 — but that included a tie in ’66. The last coach to go undefeated and untied twice in his Notre Dame tenure was Frank Leahy.

When a coach puts himself alongside that company, he’s done something.

For now, the Notre Dame coaching Mount Rushmore remains intact: Rockne, Leahy, Parseghian and Holtz. But Brian Kelly is close to creating an argument for himself. The only thing he lacks is the national championships that the others won, and that’s still on the table for this season.

At a school that has been known to grind up and spit out coaches, Kelly’s endurance and growing body of excellence are noteworthy.

“It’s a tough place,” Swarbrick acknowledged, noting that Notre Dame has never had a 60-year-old head coach. “It’s a really tough place to succeed over a long time. [Kelly] will be the first to tell you we need to win a national championship. He knows that, I know that. But what he’s done is really special.”

Brian Kelly’s decision to sub in Ian Book (above) at quarterback has paid plenty of dividends for the Irish this season. (AP)

This season has been his best work. The 2012 team was in survive-and-advance mode the entire way, winning five games by seven points or less and needed some great escapes along the way. This Notre Dame team has won consistently and at times been dominant, but rarely showing the shock-and-awe capability of the two unbeatens ahead of it in the CFP rankings, Alabama and Clemson.

The first three wins of the season were all by one score, and the offensive struggles during that time were what led to Book’s promotion to starting quarterback. There was a tense home game against Pittsburgh in mid-October. And then there was this crazy closing stretch, with the Irish hopscotching from San Diego to Chicago to New York to Los Angeles for four of their final five games.

“We’re in Los Angeles this week, I think,” Kelly deadpanned. “We’re like an on-Broadway show — we open up in a city near you.”

The winding road finished with a final challenge from a USC team that came in reeling but played a spirited and sharp first half. The Trojans’ quick passing game to the outer thirds had Notre Dame on its heels, with freshman quarterback J.T. Daniels on fire early. Defensively, USC attacked with blitzes that stuffed the Irish running game and heated up Book.

The result was a 10-0 USC lead — the first double-digit deficit the Irish have faced all year.

They shrugged it off and kept playing.

“We were staying calm,” offensive lineman Sam Mustipher said. “Eventually something was going to pop. We knew it would. Those pressures they were coming with, we knew we could make them pay.”

Notre Dame made the Trojans pay to the tune of two touchdowns longer than 50 yards — the second of which produced a 24-10 lead and iced the game. A late USC score produced a ripple of anxiety on the Irish sideline, but a recovered onside kick ended the game.

What followed was interesting. Notre Dame’s on-field celebration was pretty subdued. Other than quarterback coach Tommy Rees nearly flattening Book with a chest thump and screaming “Woooo!” there wasn’t a lot of demonstrable euphoria.

“We understand there’s so much left on the table,” Mustipher said. “We understand what’s left for us. Our ultimate goal at Notre Dame is to graduate and win a national championship. That’s still out there for us.”

This team figures to be an underdog if chalk holds and Notre Dame faces Clemson in the Orange Bowl (scouts from the Orange Bowl were plenty happy to chat up the Irish brass afterward). But they’ve got a chance. With no conference championship game to deal with next weekend, the Irish are the first team in the clubhouse with a playoff bid assured.

Which means they’ll actually watch some playoff TV this week for the first time. The weekly ESPN ranking shows on Tuesday night have been off-limits at the Notre Dame football facility.

“We didn’t watch it, purposefully,” Kelly said. “We turned it off. That was the captains’ decision. They didn’t want to see it. We’ll have it on this Tuesday.”

At a low moment two years ago in the LA Coliseum, none of this seemed possible. From flat on their backs to heads in the clouds, Notre Dame has come a long way in a hurry.

“We knew we needed to get this fixed,” Swarbrick said. “Two years later, it’s pretty fixed.”

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