Champ Bailey vividly remembers Qualcomm Stadium last Dec. 28. The Broncos had lost two games of their three-game lead over the Chargers in the AFC West but still had one more chance to redeem themselves.

Champ Bailey is pointing the way to success in Denver. (AP)

If only they could head into San Diego and come away with a win, they would finish 9-7 and head off to the playoffs as division champions.

Well, they didn’t exactly show up, especially on defense, as the Chargers ran away, 52-21. Not only did they allow their opponent to score at will and capture the AFC West title, they surrendered 491 total yards in the process.

There was finger-pointing and plenty of bad vibes to go around after a game that single-handedly ended the reign of Mike Shanahan in Denver and ushered in the Josh McDaniels era.

For all the talk this summer of Brandon Marshall and his tete-a-tete with McDaniels and the trading away of Jay Cutler, the real change this year in Denver has been on defense. The Broncos are allowing just 6.5 points per game in their perfect start through four games.

“I think what it was … by the guys we brought in, and the guys that were left here, we got rid of a lot of bad apples and we brought in a lot of good ones and I think that that was the key to really getting this thing turned around, as least positively,” said Bailey, the perennial Pro Bowl corner.

Bailey, the man Ben Watson ran down in the 2005 AFC divisional playoff game, has been around the block a few times in Denver and he knows how big the the “big play” can be on D. Bailey says that key element is back in vogue because of the fresh faces, some of whom are castoffs from other teams.

“I think it’s a combination of everything. We’ve got great leaders,” Bailey explained. “You talk about the addition of Brian Dawkins, Andre’ Goodman, Renaldo Hill, Andra Davis, these guys really … I don’t understand why their team let them go, first of all, because they’re great leaders and they’re great players as well.

“So, they just brought that attitude with them, and it’s rubbing off on a lot of guys. We had guys that had that attitude — D.J. Williams, myself, and a few others — but we didn’t have enough of the young guys believing it.”

And the proof was in the details — and stats. Only two teams gave up more than the Broncos’ 28 points per game.

Before the believing came the seeing. And new defensive coordinator Mike Nolan, fired as the 49ers‘ head coach with five games to go last season, has shown his group the way by installing a system that has more pressure on the edges with a greater dependence on linebacker play.

“I think Mike’s doing what Mike does,” Patriots coach Bill Belichick said of Nolan’s scheme. “What Mike ran at the Giants, what he ran in San Francisco, is similar to what he’s running now. It’s a little more 3-4 now, but they run 3-4, 4-3, I think all that’s overrated, but the elements of it and the fundamentals, the basics of that system are certainly evident and you can tell it’s Mike’s defense and he does what he always does.

“He’s a good game-plan coach, comes up with different things week to week that hurt the opposing offense that he’s facing. They play aggressively. They don’t make a lot of mistakes. They hit hard, they strip the ball, they turn it over, they get you in long yardage, and they make you pay. So, you don’t want to be in that situation.

Key an eye on Elvis Dumervil, who was moved to outside linebacker from an inside position in the 4-3. Belichick believes whether it’s three or four linebackers, Dumervil will be all over the field on Sunday.

“Essentially, players that play in the position that Dumervil plays, or that our outside linebackers play, there are a lot of similarities to a 4-3 defensive end, whether the guy’s hands-up or hands-down,” Belichick said.

“He plays 3-4 outside linebacker for them and they flip him — he plays on both sides. Then, when they go to their sub packages, he generally plays outside, but they have a bunch of different sub looks. They have a 2-4, 1-4, 1-5, 3-3, so they have a lot of different looks there where they bring [Robert] Ayers in the game and spin the dial there. He’s kind of all over the place, but he’s good. Generally, in the 3-4, [Dumervil] and [Mario] Haggan are the two outside guys. Sometimes it’s one of them, sometimes it’s both of them.”

The other game Bailey would really like to forget is the game last October in Foxboro, when Matt Cassel threw three touchdown passes — two to Randy Moss — in a 41-7 Patriots romp.

“Well, I think our confidence wasn’t there last year, especially in that game,” Bailey said. “I remember going down in that game, and even as you sit on the sideline you can really just tell how bad guys want it, and I didn’t feel that. It bothered me a lot, and obviously I showed that after the game.

“I hate to be the guy that sits on the sidelines and talks about it, but I feel like I’ve earned my stripes enough to really sit back and criticize people when I see things going wrong because I want to win, and I can’t have people ‘half-assing’ it, if you say, on the field, and I can’t let people get away with that.”

Attitude, according to Bailey, is the biggest change from 2008 to 2009. And with the new attitude, he doesn’t need to yell as much.

“Well, it’s funny, I don’t have to say much at all this year,” Bailey said. “I mean, we have guys that want it. That’s definitely a start.  We’ve got coaches that really want it, but when it rubs off on the players, I mean, that’s the key because we have to go out there and perform.”