No team has repeated as Super Bowl champions since the New England Patriots did it in 2004 and 2005. It's incredibly difficult to do in sports, but particularly in the NFL. Injuries happen, players come and go, and winner-take-all playoff games are much tougher than playing a seven-game series.

The Denver Broncos have a shot at winning back-to-back Super Bowls this year after riding their defense to a championship last season. This time around, they'll have a different starting quarterback than they had in 2015, with both Peyton Manning and Brock Osweiler gone.

Winning another ring won't be easy -- much tougher than it was last season -- but the Broncos are among the favorites in the AFC. Here's why they can go back-to-back, despite losing two quarterback from last year's team.

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Defense is still elite

There isn't a single person out there that will say the offense carried Denver to the Super Bowl last season. The defense, namely Von Miller, led the Broncos to the promised land in 2015, and it's going to have to do the same this season. Fortunately, the defense is still among the best in the league. Miller has emerged as arguably the best pure pass rusher in the NFL, and he showed the ability to get after the quarterback in the postseason -- specifically in the Super Bowl. The Broncos retained him for one more year with the franchise tag, and he's going to prove his worth with his best season yet in 2016.

Sure, losing Malik Jackson and Danny Trevathan hurts, but Jared Crick should help replace the former, while Brandon Marshall is an outstanding leader at linebacker and will make the loss of Trevathan hurt less. Not to mention, the secondary remains outstanding, led by Chris Harris Jr. and Aqib Talib at cornerback. The defense carried the team last season, and there's no reason to believe it can't do the same again in 2016.

They've won with mediocre quarterback play

No team in the NFL threw more interceptions last season than the Broncos. Osweiler and Manning combined for 23 picks, despite throwing just 19 touchdowns -- 28th in football. Denver was below average in just about every passing category, including completion percentage and passer rating, too. So how exactly did the Broncos win the Super Bowl with that kind of quarterback play? Because every other part of their roster was outstanding, and still is. The defense is obviously the team's best asset, but the groups of wide receivers and running backs surrounding the quarterback are strong, too.

Demaryius Thomas and Emmanuel Sanders are both atop the depth chart, meaning either Mark Sanchez or Paxton Lynch will have two talented targets. It's a quarterback-driven league, but surrounding your signal caller with playmakers is essential, and the Broncos have done that. Sanchez and Lynch are by no means strong starting quarterbacks, but they won't hold the Broncos back from making another deep run into the playoffs. Manning didn't last season, and both of these players have the potential to play better than Manning did in 2015.

Underrated offseason

Most will remember the Broncos' offseason as being one that was filled with departures. Osweiler and Jackson left, Manning retired, and Ryan Clady was traded, just to highlight a few. But it was also an offseason that saw Denver add some valuable players, and retain the most important. Holding onto Miller was huge for the Broncos' success going forward, but he wasn't the only big move. Their trade for Sanchez shouldn't be underrated as it gives them an experienced player at quarterback, and a safe option if Lynch isn't ready by the start of Week 1, which he probably won't be.

As for retaining players, C.J. Anderson and Ronnie Hillman were two key signings. While neither is Adrian Peterson or Todd Gurley, they have plenty of experience in the offense and provide a dynamic duo at running back. They will certainly help Lynch and Sanchez and take pressure off of both quarterbacks this season with their ability to handle a heavy workload. Along the offensive line, Denver replaced Clady with Russell Okung, who has dealt with injury problems of his own. But when healthy, he can be a great starting left tackle. Big-name departures obviously hurt, but the Broncos countered with smart acquisitions and re-signings.

Improved run game

Speaking of Anderson and Hillman, the Broncos' rushing attack should be better than it was in 2015. Part of that is because Denver will be able to attack downfield and open up the middle, something Manning was unable to do. But the other factor is that down the stretch last season, the run game took off. In nine of their final 12 games, including the postseason, Denver had more than 100 rushing yards. Hillman and Anderson were the main cogs in those performances, and they should be able to carry it over to 2016 behind a relatively-unchanged offensive line and a more aggressive aerial attack.

In addition to the two-headed monster of Anderson and Hillman, the Broncos now have a younger back in rookie Devontae Booker. He likely doesn't have the size or ability to be an every-down back in the NFL, but he doesn't have to be that in Denver. He just has to be a third option behind the two starters. The Broncos should climb the ranks as far as running the ball in 2016.

John Elway

The man behind all of the Broncos' recent success is John Elway, and he's still in Denver. That only means one thing: Winning will continue. He was the man behind bringing in Manning, so why shouldn't he be trusted with his decisions to trade for Sanchez and draft Lynch? Elway knows what he's doing with the team's quarterbacks, and whether it's Sanchez, Lynch or Trevor Siemian as the starter, the Broncos are going to win regardless.

Furthermore, Elway has a great working relationship with Gary Kubiak, which makes for a boatload of success on the field. And as long as Wade Phillips is heading the defense, that side of the ball will continue to shine. The trio of Elway, Phillips and Kubiak has worked exceptionally well in recent years, and there's no reason to think it won't continue. Elway has done everything right as the team's general manager, and is the main cog for Denver's winning ways.