The statuses of both players against the Pittsburgh Steelers on Thanksgiving are in doubt because of the short turnaround in between games and because they must complete the NFL's concussion protocol before being cleared.
"Well after we were all done with the locker room, both guys didn't feel right and went in the training room, and both were evaluated for concussions. Both are in the protocol," said Colts coach Chuck Pagano.
Luck was sacked twice and hit eight other times against the Titans. He is believed to have suffered the concussion when Titans defensive tackle Austin Johnson landed on top of him after recording a sack with 11:46 remaining in the game.
If Luck is not cleared, Scott Tolzien will start in his place against the Steelers. Tolzien is 56-of-91 for 721 yards with a touchdown and five interceptions in six career appearances. He has attempted only one pass since the 2013 season.
"He has a limited amount of exposure, but he is a pro," Pagano said of Tolzien. "Again, if he has to roll and be the guy, then he will do a great job. He prepares really well every single day."
After Thursday's game, the Colts don't play again until Dec. 5 against the New York Jets.
The Colts (5-5), who are on a two-game winning streak, are trying to catch the Houston Texans (6-3) in the AFC South.