It’s never a good sign when a defense allows career-long runs — by two different backs — on the first two plays on its first series.

But it was certainly a very telling sign Saturday night for Nebraska.

A recurring problem that’s popped up at various times in recent seasons for the Huskers appeared again against the nation’s top-ranked team.

Bad angles. Overpursuing. Missed tackles.

“We tackled terrible,” senior linebacker Bo Ruud said. “There were guys out of position.”

That’s bad enough by itself. Doing it against lightning-quick backs with some dazzling moves, though, means you’re in deep trouble.

Such was life for the Blackshirts against mighty USC.

The Trojans and their vaunted stable of running backs ran through, over and around Nebraska defenders for 313 yards in a 49-31 victory at Memorial Stadium.

The 8.2 yards per carry is the best rushing average by an opponent in Nebraska history.

“It’s really unexplainable,” senior linebacker Lance Brandenburgh said. “I don’t know what was going on. We all missed a lot of tackles out there, and if we want to beat the No. 1 team in the nation, we obviously can’t miss that many tackles.”

And, much like last week, Nebraska had problems adjusting to the same play the opponent ran over and over.

Last week, against Wake Forest, it was end-arounds and general misdirection that fooled the Blackshirts more than once.

This time, it was running plays up the middle. Over and over and over.

“We kind of got gutted up the middle,” senior linebacker Bo Ruud said. “I’m not sure what happened.”

The USC offensive line, starting true freshman center Kristofer O’Dowd, blew open huge holes against a Nebraska defensive front that entered the season with four new starters.

Nebraska never plugged the holes.

“We didn’t hold up very well up front,” Nebraska defensive coordinator Kevin Cosgrove said. “That was a problem the whole game.”

Were there adjustments made?

“It really was nothing that needed to be adjusted,” Cosgrove said. “It was about making plays.

“You could see the problems on the field. Changing up the fire zones wasn’t the answer. We just have to do a better job.”

To go with those wide openings, USC made the Blackshirts look silly by breaking tackles and zooming past defenders taking bad angles.

Take the first two plays of the game.

With USC backed to its 4-yard line, and a crowd of more than 84,000 roaring, freshman fullback Stanley Havili broke a Corey McKeon tackle at the line of scrimmage and ran loose for a gain of 50 yards.

The next play? C.J. Gable escaped the grasp of senior cornerback Zackary Bowman and rambled 40 yards to the Nebraska 6-yard line, setting up USC’s first TD.

Tackling problems are not a new problem in the Cosgrove regime. At least two or three games each season, it seems to be a troubling post-game topic.

Why does that problem keep arising?

“I wish I could give you an answer,” Cosgrove said. “There’s no excuse for missed tackles.

“I think, for the most part, we were in the right position, but it just seemed like there were big holes in there, big creases. You put guys in one-on-one positions with a big hole in there to make a one-on-one tackle, sometimes it could be difficult. Especially with the athleticism they have.”

The tackling problems seem contagious.

Brandenburgh overran running back Joe McKnight on a second-and-3 passing play. McKnight gained 11 yards.

“It’s all about being in a good football position, and then making the sure tackle and not going for the big hit,” Brandenburgh said. “We all need to work on that.”

On USC’s go-ahead touchdown drive, after Nebraska had grabbed a 10-7 lead, sophomore safety Larry Asante took a bad angle on Stafon Johnson, who burst through for a 16-yard gain.

Sophomore nose tackle Ndamukong Suh, regarded as perhaps Nebraska’s top defensive player, couldn’t wrap up Johnson, either.

When it appeared Suh had Johnson tackled for a loss, Johnson spun away and gained 9 yards. What would’ve been a third-and-long play turned into a third-and-1 from the Nebraska 36.

USC converted, and kept driving. Johnson followed with a 32-yard run — his career long — on the 9-play, 73-yard scoring drive that Johnson finished with an easy 7-yard touchdown run. That put the Trojans ahead 21-10 at halftime.

The missed tackles continued in the second half, and so did USC’s big running plays.

“The defense,” Ruud said, “did not show up to play.”

Reach Brian Rosenthal at 473-7436 or brosenthal@journalstar.com.