Marcus Mariota is one of the NFL’s most promising young quarterbacks, but the Tennessee Titans watched in horror Saturday as he was carted off the field with what was later announced as a fractured fibula.

The play that sent him to the locker room occurred late in the third quarter after he was sacked by Sheldon Day. Day came flying in from Mariota’s blind side and wrapped him up low, causing his right leg/ankle area to twist.

The team announced almost immediately afterwards that Mariota was ruled out for the remainder of the game, indicating the serious nature of the injury, which you can see here. After the game, Mike Mularkey told reporters Mariota suffered a fractured fibula.

This is a brutal blow, not only for Mariota but also for the Titans, who are competing to win the AFC South against the Houston Texans. Already down 25-10 at this point, this injury cast a dark cloud over an already frustrating day for Tennessee.

Veteran backup Matt Cassell came into the game to replace Mariota.