JACKSONVILLE, Fla. -- The Tennessee Titans managed all season to keep Marcus Mariota safe: with three new starters on the offensive line; with a new line coach in Russ Grimm; with revamped protection schemes; with better help from running backs and tight ends; and with Mariota's increased willingness to throw the ball away. All of this helped keep Mariota healthy.

But it took just one moment to undo all of it. Mariota tried to get out of the pocket in the third quarter, and defensive tackle Sheldon Day got him from behind and the Titans quarterback went down hard.

Jaguars defensive tackle Sheldon Day tackles Marcus Mariota from behind. Mariota suffered a broken fibula in his right leg. Reinhold Matay-USA TODAY Sports

After punching the ground once he sat up, Mariota was fitted for an air cast. X-rays showed a fractured right fibula that puts an end to his second NFL season.

What’s next for the Titans?

The regular-season finale on Jan. 1 will be the Titans’ last game if the Houston Texans beat the Cincinnati Bengals on Saturday night.

If Houston loses, next Sunday's Texans-Titans matchup in Nashville will be for the AFC South title.

For whatever is left, Matt Cassel will be the Titans' quarterback. Alex Tanney will be promoted from the practice squad to back up Cassel.

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“You never know,” Cassel said of his squad's playoff hopes. “We’re going to have to have some things fall into place. Our team is a bunch of fiery guys who are going to be resilient and go out and do their jobs. They are a bunch of professionals. I’m not worried about this locker room. The guys are going to come out and give their all, and we’ll see what happens.”

In relief of Mariota, Cassel hit on 13 of 24 passes for 124 yards. He threw a touchdown to Delanie Walker, but he had an interception returned for a 30-yard score by Jalen Ramsey and finished with a 65.3 passer rating.

Walker and running back DeMarco Murray bemoaned the Titans' failure in pass protection. Mariota was sacked twice and Cassel once.

“It’s very tough,” Walker said of Mariota’s injury. “One thing we said was protect the quarterback, and we didn’t protect the quarterback today. That’s everyone on offense.”

But they all pledged to rally around Cassel.

Dallas was 1-6 in Cassel's seven starts in 2015. He also had one start for Buffalo, a win. He completed 58.3 percent of his passes in those games, with five touchdowns, seven interceptions and a 70.6 passer rating.

“I’m very confident,” Murray said. “Cassel’s a guy who’s been around a long time. He’s won a bunch of games. He’s the next guy up. He prepares every single day, every single week, like he’s the starter. So I know he’ll be ready to play.”