Heinicke, a 2015 undrafted free agent out of Old Dominion, got into six games (one start) with the Panthers last season, completing 61.4-percent of his passes for 320 yards, one touchdown, and three picks. The 26-year-old also added another 31 yards (as well as a fumble) on five carries. He was released by the Panthers at the end of the preseason, and he’s since had workouts with the Bills and Lions.

Callahan, 26, has managed to put together a four-year career despite only appearing in a single NFL game. The former undrafted free agent out of Wesley (DE) has had stints with the Packers (two stints), Saints, Browns, Eagles, Buccaneers, and Ravens. He appeared in a single game with Green Bay back in 2017, completing five of his seven pass attempts for 11 yards. He joined Baltimore this past offseason before being cut in late August.

While Mariota wasn’t necessarily at risk of losing his starting job, he didn’t do much to quiet his critics during the Titans’ losses in Week 2 and Week 3. However, he bounced back in Sunday’s win over the Falcons, completing 18 of his 27 pass attempts for 227 yards and three touchdowns. He also added 22 rushing yards on three carries. The Titans gave up some draft capital to acquire backup Ryan Tannehill, so it’s unlikely a free agent addition would be any better than third on the depth chart. However, it makes sense for the Titans to explore some options just in case they suffer a sudden injury.