In 2015, Jameis Winston and Marcus Mariota were decorated, Heisman Trophy-winning quarterbacks entering the same NFL draft. Yet on Sunday, when the Tampa Bay Buccaneers face the Tennessee Titans in Nashville, only Winston will be starting, and he isn't entering the matchup on a high note.

Mariota now occupies a spot on the Titans' bench in favor of Ryan Tannehill, who took over as starting quarterback last week. Winston is trying to shake off a five-interception performance against the Carolina Panthers in London that raised serious concerns about his future. Neither quarterback is under contract beyond 2019.

Why is it that both have struggled?

For one thing, their offensive lines have struggled to protect them. Winston and Mariota have each been sacked 25 times this season, tied for most in the NFL. Their combined 50 sacks is more than that of all of the NFC East quarterbacks combined, according to ESPN Stats & Information data. But there's much more to it. Here's a closer look at why neither quarterback has lived up to the hype and what could be coming next.

How have Winston and Mariota been developed since they were drafted?

Laine: Although Winston has endured three head-coaching changes in Tampa Bay, he was afforded the luxury of playing in the same offensive system under Dirk Koetter and for the same quarterbacks coach in Mike Bajakian for four seasons. But unlike in Mariota's case, when Bruce Arians took over as the Bucs' head coach this year, he immediately declared that it was Winston's team. He didn't sign a high-level backup to push Winston in his final year, which the Titans did in trading for Tannehill. The previous Bucs coaching staff challenged Winston with Ryan Fitzpatrick, who wound up replacing Winston due to suspension and then performance. Arians "didn't want [Winston] looking over his shoulder, playing scared." They instead brought in Blaine Gabbert to compete with Ryan Griffin for the backup job.

Davenport: Mariota has dealt with constant change at offensive coordinator during his time with the Titans. In total, he had four coordinators and three head coaches in five seasons. It's difficult to develop the necessary comfort and confidence in a scheme when there is that much turnover. The changes have been drastic. The "exotic smashmouth" scheme that Terry Robiskie orchestrated as the offensive coordinator in 2016 and 2017 seemed to showcase Mariota the best. He reached career highs in passing yards (3,426), touchdowns (26) and passer rating (95.6) in 2016.

Tannehill was the only quarterback brought in to truly compete with Mariota. After six games, Tannehill was named the starter, and he excelled in Week 7.

What have Winston and Mariota done well in their careers?

Laine: Winston's greatest strength is his ability to keep plays alive. He makes some dazzling plays, such as when he avoided a sack before hitting Mike Evans on the move for a 64-yard completion against the Baltimore Ravens last season.

Jameis keeps the play alive and HEAVES it to Evans! pic.twitter.com/iUsCa0ko29

— Tampa Bay Buccaneers () December 16, 2018

Since 2015, Winston has thrown 30 touchdown passes from outside the pocket -- more than any other quarterback in the league. His 115.3 passer rating outside the pocket is the highest of any quarterback in that span, and his 115 first downs from outside the pocket (passing and rushing combined) in that span is the third most in the league behind the tallies of Russell Wilson and Aaron Rodgers. Despite not being the most athletic guy on the field, Winston is functionally very mobile, rushing for 65 first downs since 2015 -- 10th in the league -- though there has been some hesitation on his part to scramble in this new offense.

Davenport: After going 3-13 in his rookie season, Mariota helped guide the Titans to three consecutive 9-7 seasons. He managed to lead the Titans to a road playoff win over the Chiefs in 2017. It was Tennessee's first playoff victory in more than a decade. Mariota developed a special connection with tight end Delanie Walker over the years. The duo combined to average 71.2 receptions for 831.2 yards and 5.2 touchdowns from 2015 to 2017. Walker caught 94 passes and had his only 1,000-yard receiving season in his first year with Mariota as his quarterback. Mariota is a dynamic player when he escapes the pocket and can break long runs for big gains. He does a good job finding receivers along the boundary and throws the ball with accuracy on out-breaking routes, especially when he's rolling to that side.

Marcus Mariota turns on the jets! #TitanUp

Touchdown Titans! #TENvsHOU pic.twitter.com/Q5TqOkB08P

— Tennessee Titans () October 1, 2017

What have Winston and Mariota struggled with?

Laine: The flip side to Winston's ability to keep plays alive is that he doesn't know when to give up -- whether to take a sack or throw the ball away. Since 2015, no player has turned the ball over more times (87) or thrown more interceptions (68) than Winston. He threw five interceptions and lost a fumble against the Panthers in London in Week 6.

Beastly moves by Javien Elliott! pic.twitter.com/93rqafp7I4

— Carolina Panthers () October 13, 2019

While it would seem more likely that an interception-prone quarterback would struggle against blitzes, that has actually been a strength for him, whereas he has often struggled against teams that deploy more help into coverage (with the exception of the most recent game against Carolina, in which three of his picks came on blitzes).

Defenses such as the Cowboys have had success against Winston by keeping him in the pocket, minimizing his escape routes. At times, he has failed to account for linebackers dropping into coverage. There have been multiple instances when he put too much heat on his fastball and overthrew guys or threw a little too high over the middle, resulting in an interception. The 49ers this season had him dancing frenetically; he couldn't set his feet.

Davenport: Many of the areas in which Mariota needed to improve as a pro prospect remain issues in his fifth NFL season. He still has a tendency to wander into sacks. He is still learning to manipulate the pocket and has a tendency to focus his eyes on the pass rush instead of keeping them down the field to see receivers. He hesitates to believe what he sees.

Make way for Denico Autry. #INDvsTEN pic.twitter.com/r5TCgzhlWL

— Indianapolis Colts () September 15, 2019

But the main problem with Mariota is his inability to let the ball rip when he needs to. There's an unnecessary degree of hesitation to his game, which causes a delay in getting the ball to receivers in tight windows. Only 15.1% of Mariota's passes in 2019 were considered tight-window throws, according to NFL Next Gen Stats. Mariota tends to be too safe with the football instead of throwing with anticipation and trusting his receivers to get to the spot. Being overprotective of the football has played a role in his increased number of sacks this season. Through six games, Mariota had been sacked a league-high 25 times. He also tends to panic in the pocket, even when there is no pressure. There are times when he flees the pocket prematurely instead of standing firm and giving his receivers a chance to get open.

What will happen with each quarterback next year?

Laine: Although there has been some speculation, any reports suggesting that the Bucs will be moving on from Winston are premature. Sources inside the building say they haven't given up on him. It's too early, with just six games under Arians, and three weeks ago, Winston was touted for playing the best football of his career. Are they concerned, though? Yes. They thought they were past performances such as Week 1, when he threw three picks against the San Francisco 49ers, and Week, 6, with those five interceptions against the Panthers. That has caught them off guard. Arians said he did not consider pulling Winston in London. When asked if he was contemplating making a change during the bye week, he responded, "Absolutely not." But he also said, "If it happens again, it'll concern the hell out of me."

Arians has given Winston a clean slate this season, judging him only on what he has done through six games in this offense, but this roller-coaster ride of games ranging from "truly epic" to "downright dreadful" is not new for Winston, who has been outplayed by backups Teddy Bridgewater and Kyle Allen this season in head-to-head matchups. The key is finding a way to steady him and finding a happy medium in which he doesn't feel as if he has to be Superman on every play but can still deliver a winning performance when Evans is double-covered, when protection wanes or when the Bucs' young secondary struggles. If Arians, known as a "quarterback whisperer," can't get it out of Winston this season, can anyone? It should also be noted that the Bucs don't have to make a decision after this season on a long-term extension. They have considered using the franchise tag on Winston.

Davenport: It is highly unlikely that Mariota will remain with the Titans after the 2019 season. The team picked up his fifth-year option and added several starters in the offseason in an effort to get a clean evaluation of him. The Titans added receivers Adam Humphries (free agency) and A.J. Brown (No. 51 pick) in addition to attempting to shore up the offensive line by adding free-agent guard Rodger Saffold and using a third-round pick on guard Nate Davis. The team offered some continuity for Mariota by elevating former tight ends coach Arthur Smith to offensive coordinator to replace former coordinator Matt LaFleur.

But the franchise pulled the plug on Mariota after six games this season, which included a two-game stretch in which the Titans scored only seven points. He will likely become a free agent after this season. There could be interest in Chicago, according to Bears reporter Jeff Dickerson, depending on how well Mitchell Trubisky plays the rest of the season. Bears offensive coordinator Mark Helfrich was Mariota's head coach in college at Oregon.

The best landing spot for Mariota might be with a team such as the Eagles or Chiefs as a backup on a one-year deal. Then he could get the right tutor before hitting the market again in 2021.