Set to face the Steelers just three days from now, the Indianapolis Colts have a major problem under center.

Coach Chuck Pagano announced Monday that quarterback Andrew Luck is mired in the league’s concussion protocol on the heels of Sunday’s 24-17 win over the Tennessee Titans.

Pagano told reporters that the head injury took place sometime in the fourth quarter, putting Luck’s status in doubt for Thursday’s Thanksgiving showdown with the Pittsburgh Steelers. Pagano wouldn’t speculate on whether Luck will be cleared before the game.

“That’s up to our docs and trainers to monitor that,” Pagano said.

The Colts are planning to start Scott Tolzien, a 29-year-old career backup with just two NFL starts to his name, a source with knowledge of the situation told NFL Network Insider Ian Rapoport. Beyond throwing one pass last season for Green Bay, Tolzien hasn’t seen regular-season action since 2013.

The severity of Luck’s condition has yet to be determined, but it’s extremely rare for a player to bounce back from a concussion during a short week. It’s worth noting that Packers receiver Davante Adams entered concussion protocol after a Sunday game in Week 6 only to suit up for Green Bay’s Week 7 appearance on Thursday Night Football. Head injuries, though, are a player-to-player proposition.

Zak Keefer of The Indianapolis Star pinpointed one potential source of Luck’s injury:

Resting alone in second place behind the Texans (6-3) in the AFC South, the Colts (5-5) can hardly afford to take on the Steelers or anyone else without Luck. The team is built entirely around his massive upside and doesn’t stand a chance in the AFC without him.

Luck’s injury will be monitored more heavily than Aunt Marjorie’s turkey in the lead-up to Thanksgiving night.

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