ENGLEWOOD, Colo. -- Sustainability.

That's the key word for any NFL team in an era of knee-jerk firings and social media badgering. It's especially applicable to Super Bowl winners that have questions about whether they have the makeup to win it again -- or, at the least, be in the mix.

It's hard to stay in the mix, but it's something always made easier by having a franchise passer, which is why as the Denver Broncos opened training camp here Thursday, sustainability is key.

The Broncos had a defense that was historically good in 2015, a top-ranked unit that carried them to a Super Bowl victory without a great offense. That offense was led by Peyton Manning on his last legs, with an arm that seemed to struggle at times to make the necessary throws. When he was hurt, Brock Osweiler came in to start seven games, but it was still the defense that carried the team.

The offense wasn't very good, and that's being kind. It was actually middling, ranking 19th in points, 16th in total yards and 14th in passing. In an era built for wide-open offenses, Denver's just seemed to plod along.

"As much as we like to say we did the right things offensively, we hung on offensively," Broncos general manager John Elway said.

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Elway has a tough task ahead in his quest to repeat. USATSI

Elway was a star quarterback. So he knows a thing or two about sustainability and how the franchise passer can make it so. Without one, even with a great defense, it's tough to do.

Some will say Seattle has done a nice job of doing so, but Russell Wilson has made big strides as a passer. He's also far better than what the Broncos will trot out under center in 2016 -- no matter who it is.

With Manning retired, the Broncos hoped to bring back Osweiler, who did some good things in his place last season. But Osweiler didn't want to come back -- he was said to be angry that he went to the bench when Manning got healthy -- and he took an $18 million-per-season deal from the Texans.

That left the Broncos to sign veteran Mark Sanchez and then make a bold move to move up in the first round to take Paxton Lynch from Memphis. On the staff is also strong-armed, big-talking Trevor Siemian, a second-year passer who made some nice throws Thursday.

"The three guys competing, I feel good about them," Elway said. "Now we're not talking about Tom Brady or Aaron Rodgers. But we're good enough around them as we saw last year. It's not the mentality that you can't do it without the quarterback."

Manning wasn't Manning last season, but he could still get his team into the right plays and make sound decisions. He was also good enough in the postseason, thanks in large part to the dominant defense.

But if history is an indicator, the Super Bowl-winning formula of a dominant defense coupled with a middle-of-the-road offense doesn't work the next season. The margin for error is too thin.

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Sanchez is considered the favorite to start. USATSI

Aside from the Broncos, there have been three Super Bowl champions since 2000 who had the top-ranked defense and saw their offenses ranked in the teens in yards, points and passing.

There were the 2000 Baltimore Ravens, the 2002 Tampa Bay Buccaneers and the 2008 Pittsburgh Steelers. Of those three, only the Ravens returned to the playoffs the next season, and that was as a wild-card team.

The combined record of the three teams was 26-22, with Tampa Bay going 7-9, the Steelers 9-7 and the Ravens 10-6. They weren't awful, but they couldn't parlay their defenses to a second title.

I mentioned that to Broncos safety T.J. Ward on Thursday after practice.

"Is that right?" he said. "That sucks. That's not us. That's them."

This is the same Ward who recently told MMQB that the Broncos could have George Bush play quarterback and win. When asked about that line, he laughed it off.

"Did I say that? I didn't say that," he said. "Probably could. I made that statement because that's how much confidence I have in our defense. We have three guys capable of starting. Wasn't [a] statement against them."

It's also a statement about how confident this Denver defense is right now. They lost key players in Malik Jackson (Jacksonville) and Danny Trevathan (Chicago) in free agency, but they also have a lot of young players ready to do more, guys like pass rusher Shane Ray and corner Bradley Roby.

"The way we play is totally different than a lot of teams play," corner Chris Harris Jr. said. "They want to put up 40 and run the score up. That's not our way of winning championships. If we play our game, slow it down, run the ball, play great defense, we're cool with that. Our style is different than most teams."

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Kubiak is ready for the offense to 'get to work.' USATSI

Broncos coach Gary Kubiak responded to the question about sustainability like I expected a former quarterback would.

"I am going to tell you this: I plan on us being a good offensive football team," Kubiak said. "We battled our asses off to get to the middle of the pack last year. We turned the ball over way too much. We have some firepower on this team, guys who can make big plays. Whoever ends up being our quarterback needs to get them the ball.

"I have the belief good defense consistently wins. I think being physical and running the ball wins. I am never going to change that. I want to be the best offense we can be. We have the ability to be up there. But we have to get to work on that."

The first duty will be picking the starter. Indications are it is likely going to be Sanchez, but if he flubs in the preseason don't be shocked if Siemian gets a shot. Both did some good things Thursday, but it's clear Siemian's ball gets to the receivers quicker. Lynch was wild in his first camp practice, and will likely spend this season watching and learning.

"If we have to punt the football, we're cool with that," Harris said. "Offensively, there is no way we can be worse than we were last year."

Maybe not. But without the star under center, the debate about sustainability is there.

"The margin for error is smaller," Elway said. "You have to make plays. The great quarterback will cover up a lot of holes. No question."

"We know what we're capable of doing on defense," linebacker Brandon Marshall said. "We showed last year what we could do if we play great defense."

Maybe so, but history shows that it might take a lot more than that to get back. The Denver offense has to be better.