NASHVILLE, Tenn. -- What happened to Marcus Mariota? This was supposed to be his superstar year. The year he would finally stay healthy. The year he would take the next step and join the next generation of elite quarterbacks. The year every NFL defense would fear playing the dangerous, silent assassin.

But as Mariota underthrew Harry Douglas on a crossing route Sunday for his sixth interception in two games, it finally sank in that this probably won't be his superstar year. Mariota, who is certainly the Tennessee Titans' franchise quarterback, already has a career-high 12 interceptions compared to nine passing touchdowns. His 79.1 passer rating ranks 29th in the NFL.

The Titans are 7-4, winners of five of their past six, and lead the AFC South. But their defense, not Mariota, has been the primary reason.

It's not that Mariota has been terrible. He leads the NFL with four fourth-quarter or overtime comebacks this season. That has helped the Titans just about taste their first playoff appearance since 2008.

But Mariota hasn't had the Year 3 that many of us expected. After two solid, injury-shortened seasons, Mariota appears to have taken a step back instead of forward. Let's examine why.

First, Mariota is not Russell Wilson. And that's OK, because there are very few NFL quarterbacks who can transcend an offense like Wilson. Mariota hasn't proven, with consistent accuracy and health, that he can put the entire team on his shoulders quite yet.

Mariota's struggles have shown us that he needs solid contributions from those around him to be a potential superstar-level quarterback.

But when diving into what's gone wrong with Mariota, you should first look at what's going on around him.

The Titans were built to be the smashmouth offense that had a ton of success last season: run first, with an elite offensive line paving the way for DeMarco Murray and Derrick Henry to power through defenses. An infusion of receiver talent had many believing Mariota finally had the weapons to make the Titans a complete offensive juggernaut.

Instead, the run game has taken a major step back. The Titans have rushed for fewer than 100 yards in seven of their 11 contests. They had four such games last season. The offensive line has regressed and struggled, particularly in the middle. Injuries have sapped Murray's burst. The Titans also did not have an adequate replacement for blocking tight end Anthony Fasano, who left for Miami in free agency, but still have a scheme meant for him.

The results have been ugly, such as last week, when Delanie Walker pulled to block Colts nose tackle Al Woods (predictably didn't work) or when Jonnu Smith and Phillip Supernaw lost some one-on-one battles against blitzing Steelers linebackers the week before.

The rookie receiving additions -- Corey Davis and Taywan Taylor -- have made some mistakes, Eric Decker hasn't made a big impact, and there's a speed-and-separation element missing from the position groups.

Walker has repeatedly said the blame can't all be put on Mariota, and that is true. Execution could be a lot better throughout the offense.

Put that all together and it makes sense as to why Mariota appears to be undergoing some of the same growing pains that Cowboys QB Dak Prescott is realizing without Ezekiel Elliott.

The Titans' offense is also a contrast to a lot of the spread schemes Mariota ran at Oregon. It's not college anymore, and Mariota has had a lot of success in this scheme (26 TDs to nine interceptions in 2016), but there are times when he looks uncomfortable. Titans coach Mike Mularkey loves multi-tight end sets and condensed formations. That works well when the run game is churning, but when it doesn't, it leads to one- or two-route combinations.

To Mariota's credit -- and detriment -- he hasn't been shy firing into tight windows. He makes at least one throw a game that deserves consideration for pass of the year, like his beautiful 37-yard drop-in-the-bucket pass to Walker against the Colts despite trailing triple coverage and a defender over the top.

Marcus Mariota just dropped a dime to @delaniewalker82 👀#TitanUp pic.twitter.com/C7dmvxJdql

— Tennessee Titans (@Titans) November 26, 2017

But a combination of ambitious routes and a receiving corps of primarily possession receivers has forced Mariota to be consistently precise with his accuracy. When he hasn't been, interceptions often occur. Mariota hasn't played nearly as bad as his touchdown-to-interception ratio would indicate.

Easy throws early in the game could help develop a rhythm. Getting Mariota more run-pass options, which have been successful, seem to be an ideal way to do that. Mariota also is a play-action savant. The Titans have increased the use of play-action recently, but they can still utilize it more. Mariota is 11th among NFL QBs with 86 play-action dropbacks (Wilson and Case Keenum lead the NFL with 108 and 105, respectively).

That's part of the issue, but it's not the whole story. It's an organization's job to make sure its franchise quarterback is in the best position to succeed, but that quarterback also holds responsibility in stepping up.

Mariota's accuracy has been a roller-coaster this season. His recent string of interceptions -- eight in the past four contests -- is the culmination of being randomly off on a handful of passes each game.

"It's just coming down to throwing. I'm missing -- either I'm sailing it or leaving the ball behind," Mariota said. "I gotta find ways to improve. I gotta get better. I can't keep hurting this team. And I will -- I'll definitely get better at it."

Those are great words to hear for Titans fans because that sounds like a man who truly understands that his issues can't solely be blamed on the scheme or players around him. It's also a huge positive that Mariota continues to bounce back and show great confidence and resolve after mistakes. On the deciding fourth-quarter drive against the Colts, Mariota completed a clutch tight-window throw to Davis after two earlier interceptions.

"There's a lot of guys that can't overcome those," Mularkey said. "I've yet to see him let that affect his play."

There's some thought that Mariota's recent injuries (Week 4 hamstring strain, late 2016 broken leg) might be impacting his mechanics. A weakened hamstring can affect a quarterback's balance and footwork. But Mularkey said Mariota is healthy, and the quarterback won't use his weekly bumps and bruises as an excuse for his play.

His recovery from the broken leg likely had an impact on his progression because he didn't get to spend his offseason improving footwork as he was trying to rehab.

This past offseason, Mariota and Titans quarterbacks coach Jason Michael identified footwork, rhythm and ball security as three areas to seek the most improvement.

Mariota still has a tendency not to set his feet when throwing. He also has a tendency to arm-throw instead of stepping into throws and generating power from his lower body. This occurs when he's pressured and when he has a clean pocket.

The biggest example of this has been his puzzling overthrows, such as the one he threw 10 yards over Rishard Matthews' head for the first of four interceptions against the Steelers. Mariota has 42 overthrown passes, per ESPN's Stats & Info, and four of those have turned into interceptions. Mariota had just two overthrown interceptions in his first two NFL seasons.

These appear to be fixable issues and Mariota is just 24, so these struggles are not unusual. It would be an overreaction to say he's broken or not fit to be a franchise quarterback. But he has slumped significantly in Year 3. The question now is if he can get out of it in time to help the Titans potentially make noise in the playoffs.