ESPN Colts reporter Mike Wells says that Andrew Luck will need to go through several stages of the concussion protocol and breaks down how Scott Tolzien can expect to play Thursday against the Steelers. (1:40)

Luck needs to go through several steps to be cleared (1:40)

INDIANAPOLIS -- Colts quarterback Andrew Luck has officially been ruled out of Thursday's game against the Pittsburgh Steelers because of a concussion.

Luck didn't practice all week after suffering the concussion on a sack in the fourth quarter of Sunday's game against the Tennessee Titans.

Luck has to clear the NFL's concussion protocol before he's allowed to play.

Speaking Wednesday, Titans coach Mike Mularkey said he was surprised Luck was not pulled from Sunday's game with the concussion.

"Very surprised, based on how important it is to the NFL that they protect our players," Mularkey said.

The Colts' next game after the Steelers is Dec. 5 at the New York Jets.

Luck played in the first 51 regular-season games of his NFL career. But Thursday will mark the 10th game that he has missed since Week 4 of last season.

Scott Tolzien is expected to start in Luck's place against the Steelers. Tolzien is 56-of-91 for 721 yards, a touchdown and five interceptions in six career appearances. His last meaningful playing time was against the Minnesota Vikings on Nov. 24, 2013, when he was with the Green Bay Packers.

The Colts elevated Stephen Morris from the practice squad Wednesday to back up Tolzien.

Colts starting safety Clayton Geathers (concussion) also has been ruled out of Thursday's game. Wide receiver Donte Moncrief, meanwhile, was a late add with a hamstring injury and is questionable.

The Colts (5-5), who are on a two-game winning streak, are a game behind the first-place Houston Texans in the AFC South. The Steelers have beaten Indianapolis in each of the past two seasons by a total of 52 points.

Information from ESPN's Paul Kuharsky was used in this report.