Ryan will miss his first game since the 2009 season, when he was sidelined for two straight December games with turf toe. The former league MVP had made 154 consecutive starts before the ankle injury.
Falcons coach Dan Quinn listed Ryan as questionable and said the quarterback would be reevaluated after returning to practice in a limited fashion on Friday. The team determined it was best to sit Ryan.
Danny Etling has been promoted from the practice squad to serve as Schaub's backup on Sunday, with the team releasing offensive lineman John Wetzel in a corresponding move.
The 1-6 Falcons, losers of five straight, saw Ryan go down in the fourth quarter of last Sunday's 37-10 loss to the Los Angeles Rams. The injury occurred when Ryan was sacked by Rams star Aaron Donald, who beat Falcons guard Wes Schweitzer on the play.
Ryan, who wore a boot immediately after the game, was spotted in the locker room Wednesday walking without the boot, although he did not practice Wednesday or Thursday.
Quinn declined to reveal the severity of Ryan's sprain and would not refer to it as a high ankle sprain. Ryan will have extra time to rest with the bye week following Seattle. The Falcons begin NFC South play Nov. 10 at New Orleans following the bye.
Schaub, 38, last started a game in December of 2015 while with the Baltimore Ravens. The two-time Pro Bowler has 92 career starts and a career record of 47-45. He is 1-1 against the Seahawks.
"I have to do my job, and that is my role for this team,'' Schaub said earlier in the week. "If I am called upon to go out there and play, then I have to perform my role as the quarterback. I have to lead the offense, get completions, know game situations, convert third downs and score in the red zone. Football doesn't change. We just have to go out there and execute our plan."
Schaub's lone career win against the Seahawks came on Dec. 13, 2009, when he passed for 365 yards and two touchdowns for the Houston Texans in a 34-7 victory. On that same day, Ryan missed his last NFL game while inactive against the New Orleans Saints due to turf toe.