CHARLOTTE, N.C. -- Pro Football Hall of Fame executive Bill Polian has a theory: Super Bowl-caliber teams typically stay together and are functional for six years at best.

“Reaching the Super Bowl is the culmination of that climb and natural decline can set in soon after," Polian said.

For the Carolina Panthers and Denver Broncos, it was more like falling off a cliff following the 2015 season after meeting in Super Bowl 50.

Both teams are 4-8 heading into Sunday’s game at Bank of America Stadium and on the verge of a fourth losing season in the five years since Denver won the title with a 24-10 victory at Levi's Stadium in Santa Clara, California.

The Panthers are 33-43 since going 15-1 during the 2015 regular season. The Broncos are 31-45 since going 12-4 that year.

Carolina’s 2017 trip to the playoffs, which ended with a loss in an NFC wild-card game, is the only postseason appearance between the teams during that time.

The common denominator other than losing? Declining quarterback play.

Cam Newton was the league MVP for Carolina in 2015. Manning was in the last season of his Hall of Fame career in Denver.

Since then, the Broncos have had nine different quarterbacks start at least one game -- 10 if you count Kendall Hinton after starter Drew Lock and both backups went on the NFL’s reserve/COVID-19 list two games ago. The Panthers have had seven quarterbacks in that span.

The other teams with similar turnover since 2015 are the New York Jets (8), Washington (8), Chicago (7), Miami (7) and Cleveland (7). None of these teams have a cumulative winning record the past five years and none are better than Miami at 36-40.

For Denver, the downward spiral began when Manning retired after winning his second Super Bowl. Newton’s decline was more gradual and painful, as a shoulder injury in 2018 and Lisfranc injury in 2019 led to his release this past offseason.

According to ESPN Stats & Information research, the Panthers rank 26th in Total QBR since 2015 and the Broncos 31st.

ESPN reporters Jeff Legwold (Broncos) and David Newton (Panthers) take a closer look at what happened:

What was the biggest factor in their downfall?

Legwold: The first real sign of trouble came about 12 weeks before the Super Bowl. Manning injured his foot. He missed seven starts but played every playoff game, including Super Bowl 50. Still, any sliver of hope Manning would play even one more season limped off the field with him in Week 10 after his fourth interception.

Since Manning's retirement, the Broncos have had a revolving door at quarterback.

Beyond that, the 2015, 2016 and 2017 drafts yielded little -- as in very little -- long-term help, other than safety Justin Simmons and left tackle Garett Bolles. Of the Broncos' 25 selections in those three drafts -- players now in their fifth, sixth or seventh seasons -- four are on the current roster and have been with the team since they were selected.

Of the team's first-round picks in 2014, 2015, 2016 and 2017, only Bolles was signed to a second contract.

Newton: This from former Panthers coach Ron Rivera, who was fired with four games remaining in the 2019 season, sums it up. "The biggest thing that happened the last two years that contributed to this long-term mediocrity was we had no quarterback situation settled," he said. "If you have a settled quarterback situation, you're pretty doggone good. Our problems didn't start until when? Until our quarterback got hurt."

Rivera wrote off the 6-10 2016 season to the Super Bowl hangover. The Panthers bounced back to go 11-5 in 2017 and were 6-2 midway through the 2018 season with Newton playing arguably better than his MVP season.

Then came the shoulder injury that led to a 1-7 finish to the season, 0-6 for Newton before he was shut down. Two games into the 2019 season, Newton suffered a Lisfranc injury that basically ended his career in Carolina with eight straight losses.

Current starter Teddy Bridgewater has brought stability to the position, but with running back Christian McCaffrey missing nine games to injury and an overhauled defense, that hasn’t led to wins.

What was their biggest personnel mistake?

Legwold: The Broncos' past three draft classes have brought youth, athleticism and some hope, but the roster doughnut created by the 2015 though 2017 drafts is still being felt. And no draft misses sting worse or longer than a miss on a first-round quarterback.

In 2016, the Broncos moved up in the first round to select Paxton Lynch -- the first draft in the months that followed the Super Bowl win. They’re still trying to recover.

The Broncos signed Case Keenum as the bridge quarterback in 2018, but when Lynch couldn’t emerge from his third training camp as even the backup, he was released. He has not appeared in a regular-season game since and remains unsigned. In the end, he made four starts and threw four touchdown passes to go with four interceptions.

Newton: Let’s start with giving cornerback Josh Norman the franchise tag in 2016 and then rescinding the tag when a long-term deal couldn’t be reached. Norman was a key piece of the 2015 defense, led by middle linebacker Luke Kuechly. That sent the wrong message to a locker room in which Norman was a popular player.

Then there were poor drafts in 2014 and 2016. Only safety Tre Boston, a fourth-round pick in 2014, remains on the roster and Carolina didn’t re-sign him in 2017 before bringing him back in 2019.

Age also was a factor, as many veterans such as tight end Greg Olsen and center Ryan Kalil became injury-prone and saw their performances drop.

Then there was the change of ownership amid allegations of sexual and workplace misconduct by team founder Jerry Richardson.

It was the perfect storm of what could go wrong did go wrong.

What's the blueprint to get back to a Super Bowl level?

Legwold: The Broncos are positioned well against the salary cap in 2021 -- even a reduced cap due to COVID-19 -- and they have one of the younger rosters in the league. They uncovered plenty of stars at the offensive skill positions in the past three draft classes. They’re more athletic up and down the roster, have more speed and have started to reclaim some depth.

Beyond changing quarterbacks so often, the Broncos have used five different offensive coordinators and five different quarterbacks coaches in the past five years. They can't keep changing playbooks, playcallers, terminology and quarterbacks every year and wonder why they can't score touchdowns.

Eventually a quarterback has to hold up in the job when it comes to pressure, criticism and just plain good decision-making in the heat of the moment.

If Drew Lock is that guy, then they're a lot closer to getting back in the postseason conversation than if they have to dive into the top of the draft again.

Newton: Offensive coordinator Joe Brady has called Bridgewater a “franchise" quarterback. But is he really? Or is he just a bridge to the next franchise quarterback?

If it’s not Bridgewater, then the Panthers need to use a top-10 pick in this year’s draft to find their next Cam Newton. The offense, otherwise, is in good shape moving forward if a long-term left tackle can be found.

There’s a lot of young talent on the defense, starting with first-round pick Derrick Brown at tackle and second-round pick Jeremy Chinn at linebacker/safety.

With a few more pieces on defense behind the leadership of first-year NFL coach Matt Rhule, the Panthers seem to be trending in the right direction.

This young group just needs to learn how to win. The 2015 team went 7-1 in one-possession games. This year’s team is 2-7.