If USC's sudden defensive breakdown after three games seems shocking, it shouldn't be.

Not if you carefully studied Steve Sarkisian's coaching resume. Not if you observe him on the practice field, where he spends almost 95 percent of his time with the offense. Not if you watched as he made little, if any, halftime adjustments as Boston College trampled through the Trojans for so many rushing yards, you'd have thought it was Oregon on the other side of the line of scrimmage.

That wasn't just another defeat in Chesnut Hill on Saturday night. It was among the more embarrassing losses in recent school history.

USC teams do not get outrushed 452 yards to 20. It just isn't supposed to happen.

But it did. It was as real as all that BC emotion flooding out from the stands. A small, unranked bunch beat all those four and five-star Trojans recruits into the ground. The same team that had been pounded for 302 yards rushing by Pittsburgh a week earlier made USC's offensive and defensive lines look thinner than your average lobster roll on the way to a 37-31 upset that was far worse then the final score indicated.

Maybe this is what happens when you hire a head coach with little or no defensive background. In 2011, in Sarkisian's third year at Washington, his team finished 105th or lower nationally in scoring defense, pass defense and total defense.

Eventually, Sark, whose entire coaching career has been on the offensive side, hired Justin Wilcox as his defensive coordinator, and things improved dramatically. But Wilcox came with him to USC, and after three games, this Trojans' defense is seriously staggering.

First, Stanford gashed it for 413 yards, failing to win the game only because it couldn't convert inside the red zone. Then this, a Trojans' performance so ragged, it couldn't be blamed solely on a post-Cardinal letdown, or a long cross-country plane ride.

You don't stink up the place like that, coughing up 506 total yards, unless you have some serious problems. And a defense that was supposed to be the best in the Pac-12 and one of the better units in the country suddenly has more holes in it than a bad summer movie script.

How could Boston College make Leonard Williams & Co. look so confused with their read-options? Where were the USC defenders on the edge? And how in the world did they turn tiny quarterback Tyler Murphy into looking like the second coming of Marcus Mariota?

Offensively, most of the problems centered on the Trojans' young blockers, who were clearly overmatched. They couldn't run block, and they couldn't pass block. Other than that, they were fine.

Not that Sarkisian helped much with his play calling. Even after it became clear Javorius Allen and Justin Davis couldn't even get to the line of scrimmage most of the time, Sark kept calling run plays, especially on first down.

Whatever happened to taking what the defense gives you? Clearly, BC was concentrating on stopping the run and giving USC the intermediate passing game, but Cody Kessler, who suffered five sacks, wasn't able to take much advantage of it when it mattered.

The Trojans finally ditched their short, horizontal passing game to rally some with 14 points in the fourth quarter, but every time you thought they had a chance to come back, the Eagles would get the ball back and the rest was history.

So just as quickly as the 2014 optimism skyrocketed a week ago, it has plummeted back to Earth. Instead of being 3-0 and looking ahead to bigger and better things, USC is a sagging 2-1 after losing to a 17-point underdog.

And if Murphy and the less-than-imposing Eagles can do this to the Trojans, what are Arizona State, UCLA and Notre Dame apt to do, let alone a more sophisticated read-option team such as Arizona, or a hungry Utah squad in Salt Lake City?

The road that looked so smooth and inviting after the great escape at Stanford now is filled with potential potholes again.

Longtime Trojans fans have every right to be disappointed.

But if they go back and review the new head coach's resume, they really shouldn't be surprised.