Head coach Mike Vrabel explained the decision to reporters on Wednesday:
Things couldn't have gone much worse for Mariota during his most recent outing.
He finished an abysmal 7-of-18 for 63 yards, zero touchdowns and two interceptions before giving way to Tannehill. The latter was better, going 13-of-16 for 144 yards, but he threw no touchdowns and one pick while failing to put a single point on the scoreboard.
The Titans fell to 2-4 on the season and failed to score more than seven points for the third time in four games.
This is not how things were supposed to develop for Mariota after Tennessee selected him with the No. 2 overall pick in the 2015 draft. He has looked like anything but a franchise quarterback this year with no touchdown passes in three of his team's six games. He also isn't using his legs as he has in the past and has yet to top 34 rushing yards in a single contest.
In fairness, the offensive line has been a major concern.
Mariota was sacked three or more times in five of six games, including when he was sacked a head-turning nine times during a loss to the Jacksonville Jaguars.
The O-line concerns and trouble finding the end zone have not led to the follow-up effort Mariota was looking for in 2019 after he threw for just 11 touchdowns last year and finished the 2017 season with 13 passing scores and 15 interceptions.
He has continued trending in the wrong direction since he set career highs nearly across the board in 2016 with 3,426 passing yards, 26 touchdown throws and nine interceptions.
Mariota is set for unrestricted free agency after this season on his current contract, and the Titans will eventually have a decision to make about his future as the potential face of the franchise. This decision doesn't necessarily bode well for his long-term standing with the team, especially since Tannehill will have the chance to cement his standing.
The Chargers are 2-4 as well, and Tennessee's Week 8 opponent is Tampa Bay. The Buccaneers are last in the league in passing yards allowed per game and could be the ideal matchup for the 31-year-old Tannehill to rediscover his form from the 2014 and 2015 seasons when he threw for more than 4,000 yards for the Miami Dolphins.
If he does, the Titans' quarterback job could be his for the rest of the year.