The emergency third quarterback is back.
NFL teams used to have the ability -- from 1991 through 2010 -- to have the third QB on the roster not count among the players active but able to play if a team lost its top two quarterbacks. (UPDATE: If either of the first two QBs is medically cleared to return to play, they must return over the third QB in that game.)
The rule was reinstituted this week after the 49ers lost both starter Brock Purdy and Josh Johnson to injury in last season's NFC Championship game, forcing Purdy to play when he couldn't throw at all (and having running back Christian McCaffery play a couple QB snaps too.) The irony there -- the rule wouldn't have made a difference in that game, because while the Niners did have a third QB on the roster, Jimmy Garoppolo had not practiced in weeks because of a broken foot and wouldn't have been available anyway.
It'll make for interesting roster constructs. About half the NFL teams have gone to having just two QBs on the active roster and a third on the practice squad. The new rule says the third QB has to be on the 53-man roster; a practice squad QB elevated on game day (teams get two PS elevations per game) cannot be a third QB.
It's not like it is used much. The one time I can think of it coming into play for the Cardinals was near the end of the 2005 season. Kurt Warner started 10-for-10 in a game at Houston before hurting his knee. Josh McCown was the backup, but he was suffering from illness or food poisoning and he tried to play before it became clear he could not. Enter John Navarre, the emergency third QB who ultimately finished the game in a 30-19 loss.
With Kyler Murray's injury, it would make sense to see the Cardinals with three QBs on the roster while he rehabs (and probably once he returns.) You figure Colt McCoy and Clayton Tune are two of those guys, with David Blough and Jeff Driskel probably battling for the other spot.