NASHVILLE, Tenn. -- After the first month of the season, there was a degree of optimism around Tennessee Titans quarterback Marcus Mariota. In a Week 4 win over the Falcons, Mariota completed 66.7% of his passes and threw three touchdowns.

But whatever hopefulness existed in September withered when October began. Week 5's start against the Bills was a tough game for Mariota, who was sacked five times and completed 13 of 22 passes for 183 yards. And Sunday’s performance against the Broncos was one of Mariota's worst of his career.

His 38.9% completion percentage against Denver was his second lowest, better only than a 30% performance in a Week 14 win over the Broncos in 2016. Mariota was benched in the third quarter in favor of backup quarterback Ryan Tannehill.

Here are three areas where Mariota and the offense have struggled the past two weeks:

Turnovers: After throwing a career-high 15 interceptions in 2017, Mariota had gotten better at avoiding picks: He had eight in 2018 and none through the first four weeks of 2019, when he had seven touchdowns. Avoiding turnovers led to more efficiency on offense to start the season. Unfortunately, the same can't be said for Mariota's play against the Broncos.

Mariota's streak of 156 consecutive passes without an interception was broken in the second quarter, when Broncos cornerback Chris Harris intercepted him. Titans coach Mike Vrabel replaced Mariota with Tannehill after Mariota’s second interception.

"I was inaccurate today, and I didn't give our guys a chance to make plays,” Mariota said. “It starts with me. I need to do a better job. Not being able to convert third downs, complete passes and get into a rhythm, it's on me."

Red zone woes: Against the Bills, the Titans' offense produced seven points despite making three trips into the red zone. Two red zone touchdowns were taken away because of penalties. One of the penalties came when Mariota threw the ball across his body to A.J. Brown after stepping past the line of scrimmage.

Against the Broncos, Tennessee didn't venture into the red zone until the fourth quarter. The Titans failed to score despite getting the ball inside the 20-yard line twice with Tannehill under center. Through six weeks, the Titans have averaged 2.5 red zone trips a game. Only 1.3 of those trips are resulting in touchdowns. Both averages are lower than the 1.6 touchdowns Tennessee averaged on 2.9 red zone visits last season.

"When you’re behind the sticks in the red zone, and you’re third-and-goal from the 9-yard line, it is tough," Mariota said of the red zone struggles. "At the same time, though, we have to be more efficient. When we have our opportunities, we’ve just got to go make plays."

Indecisiveness: After Sunday's game, Mariota said he told Tannehill to trust what he sees. But Mariota hasn't been following his own advice. Indecisiveness has been a significant handicap to Mariota's success.

It's not all Mariota's fault, but the Titans have allowed an NFL-high 25 sacks -- including 12 over the last two weeks. According to NFL Next Gen Stats, Tennessee's offensive linemen have actually been pass-blocking well at times, sustaining their blocks for at least 2.5 seconds 62% of the time this season -- good for fifth best in the NFL per ESPN's Pass Block Win Rate.

Part of the problem is Mariota is averaging 2.84 seconds before throwing passes, the ninth-longest average time in the NFL. And when the Titans' offensive line doesn't give up a pass block win (rusher beating a block within 2.5 seconds), Mariota's Total QBR of 42.8 ranks 29th out of 30 qualified quarterbacks. Only Dolphins quarterback Josh Rosen had a worse rating.

Vrabel said he'll decide whether Mariota or Tannehill will start Sunday after consulting with general manager Jon Robinson. That decision is expected to be made by Tuesday. Neither quarterback is under contract beyond this season.

Whoever plays will face a significant test when the Chargers bring their fifth-ranked pass defense to Nashville.