Strong safety J.J. Wilcox, right, and the Dallas Cowboys defense has struggled to keep up with opposing offenses this season while it switches from a 3-4 to a 4-3 under Monte Kiffin. (Tim Heitman/USA Today Sports)

Dallas Cowboys owner Jerry Jones, a man with a reputation built on noise — big signings, huge stadiums, a blinding light show that he sees as the NFL — couldn’t help but notice the quiet. He entered the home locker room at AT&T Stadium after his team’s 51-48 loss to the Denver Broncos on Sunday, and silence surrounded him.

“The quietest dressing room that I’ve ever been involved in,” Jones said afterward.

The low volume couldn’t have been because of shock. The Cowboys had lost to arguably the NFL’s best team, with quarterback Peyton Manning, at age 37 playing his best football. Players, coaches and Jones admitted the Cowboys weren’t taken by surprise.

Instead, it was the undeniable fact that hung in the air after Dallas’s third loss in five games — and second consecutive contest allowing at least 400 passing yards: that the Cowboys have a losing record and face an uncertain future because of a disorganized, generous defense.

“Defensively, we were just terrible. No way around it,” Cowboys linebacker Sean Lee told reporters after the game. “We were just terrible across the board. Fifty-one points, unbelievable. We need to take a look at ourselves individually and collectively and find a way, because we’re letting the team down.”

The Post Sports Live crew offers bold predictions for the Redskins game at the Cowboys on Sunday. (Post Sports Live/The Washington Post)

Dallas’s defense, in the first year of an overhaul under coordinator Monte Kiffin, has kept its team underwater. The Cowboys are a team overflowing with talent, and instead of celebrating a share of first place in the NFC East, this defense has the team on the verge of panic.

“We’ve got to find a way to be better,” Lee said, “and it’s got to be now.”

The Cowboys have allowed an average of more than 27 points per game, and they’re 31st in the NFL having surrendered 326.4 passing yards a game. This past Sunday, the defense was whistled seven times for penalties, including one the Broncos declined.

This week, they’ll host the Washington Redskins, who because of their own problems have been forced to pass more often this season than they’d prefer. Soon after Dallas’s latest loss, it was already looking toward Washington and quarterback Robert Griffin III.

“We’ve got to be ready, prepared,” said Dallas defensive lineman Jason Hatcher, who referred to Griffin as another top-five NFL quarterback. “We’ve really got to win this one.”

The Cowboys were supposed to have fixed their defense, after firing coordinator Rob Ryan following the 2012 season. Ryan’s defense, an aggressive 3-4 scheme that highlighted franchise pass rusher DeMarcus Ware, was merely mediocre last season; Dallas was 19th in total defense, but it was the clearest reason for the team’s 8-8 record.

Coach Jason Garrett brought in Kiffin, the 73-year-old architect of the Tampa-2 defense who had become available after resigning as coordinator at the University of Southern California (where Kiffin’s son, Trojans head coach Lane Kiffin, was granted a few months’ clemency before being fired late last month). When Monte Kiffin was with the Tampa Bay Buccaneers, he was seen as a brilliant innovator. But the past four years, in which he followed his son and has now become the face of Dallas’s disappointing start, have tarnished his image.

The Post Sports Live crew offers takeaways from the weekend's NFC East games and also look at whether the division's poor play means there is a still a shot for the Redskins to make the playoffs. (Post Sports Live/The Washington Post)

Now, observers wonder if second-year cornerback Morris Claiborne and linebacker Bruce Carter, who each have lost their starting jobs, have struggled to master the zone concepts and assignments within Kiffin’s scheme.

Perhaps more glaring, the question exists if Ware’s immense talents are being wasted following a move from outside linebacker to defensive end to fit Kiffin’s base 4-3 design. Ware has four sacks in five games, still on pace for nearly 13 this season, but all four came in two games. He was unable to pressure Manning on Sunday and was treated for cramps.

Jones said after Sunday’s loss, in which his team allowed 517 total yards, he would “cut (Kiffin) some slack” because it was Manning and the 5-0 Broncos who’d victimized the Dallas defense.

A few days before the loss to Denver, though, Kiffin admitted his group needed to show improvement and that linebackers are still learning coverage responsibilities.

“We’ve all got to do a better job,” he told reporters . “We’ve got to do a better job as coaches; got to do a better job as players. . . . We’ve just got to hang in there and keep doing it.”

This past Sunday, in a game that Jones said made his team and fan base “sick,” the greatest source of the nausea was that the defense ruined veteran quarterback Tony Romo’s best game as a pro. He broke Don Meredith’s 50-year-old team record with 506 yards passing against the Broncos — who, by the way, are the only team that has surrendered more passing yards — and led his offense to their highest scoring output ever in a loss.

“You’ve got to give the offense an unbelievable amount of credit,” Lee said. “They scored 48 points, and the fact that we weren’t able to win when the offense scores 48 points is absolutely unacceptable.

“We have to find a way to be a better defense. Right now we’re not a good defense. . . . Until we get better, we’re not going to win ballgames. It’s purely on us.”