Tennessee signed Cassel last offseason to serve as Marcus Mariota's backup, which is a role he is now poised to maintain for the next two years.
After Mariota suffered a leg injury in the Titans' second-to-last game of the season, Cassel started in Week 17.
He threw for 150 yards, one touchdown and one interception in a winning effort against the Houston Texans.
Mariota underwent surgery to repair his fractured right fibula in December and was expected to need four-to-five months of recovery time.
That means he should be ready for the 2017 season with plenty of time to spare, but Cassel provides some insurance.
Cassel's best season came in 2010 with the Chiefs when he threw for 3,116 yards, 27 touchdowns and seven interceptions en route to his only Pro Bowl.
While Cassel won't be used in game situations unless needed, having someone with his level of experience behind a developing quarterback in Mariota provides the Titans with a solid contingency plan.