JACKSONVILLE, Fla. -- The Tennessee Titans didn't feel comfortable with Marcus Mariota starting at quarterback against the Jacksonville Jaguars in Week 4, but Mariota came on in the first quarter after starter Blaine Gabbert was sacked and left the game.

Gabbert was later ruled out of the game with a concussion. Mariota and the Titans won Sunday's game 9-6 to hand the Jaguars their first loss of the season.

Blaine Gabbert was knocked from the game on this sack by Calais Campbell. AP Photo/Stephen B. Morton

Head coach Mike Vrabel had opted to start Gabbert, with Mariota still not fully recovered from the pinched nerve in his elbow.

Their plan was foiled on their third drive when Gabbert was sacked by Jaguars defensive tackle Malik Jackson on third-and-19.

Gabbert fumbled and Jacksonville recovered. Jackson was called for lowering his head, giving the Titans the ball back and a first down. Tennessee was forced to turn to Mariota.

Mariota came in and handed the ball off to Dion Lewis on his first snap. On the next play, he pitched the ball to Lewis for a 9-yard rushing gain. Mariota's first pass attempt, intended for wide receiver Rishard Matthews, fell incomplete. Ryan Succop connected on a 39-yard field goal to make the score 3-0.

Mariota came back in on the next series with the Titans having the ball at their 4-yard line. Despite being under pressure, Mariota managed to escape the pocket and scramble for a 13-yard gain. He then picked up four yards a couple of plays later before the first quarter ended.

Mariota finished the game 12-of-18 passing for 100 yards and also rushed seven times for 51 yards.