In the modern NFL, it's quite common for teams to only carry two quarterbacks on the active roster. If they both go down, usually punters or other specialty players are prepared to fill in.
In the case of the New England Patriots, they're fortunate to have a former QB playing wide receiver for them.
Julian Edelman, one of the best pass catchers for the Pats over the last few years, played quarterback at Kent State, and on Thursday night he'll be just one play away from going under center.
With Tom Brady still serving his suspension and Jimmy Garoppolo likely out with a sprained AC joint, rookie Jacoby Brissett is set to start Thursday night's contest with the Houston Texans. With no other backups, Edelman is ready to slide into that role.
From the NFL Network's Ian Rapoport:
The Patriots have done extensive preparation over the past few days to get WR Julian Edelman ready as the backup QB. It may actually happen.
With such a short week, it would be tough for a veteran free agent to come in and learn the playbook in time. Even with Brissett, head coach Bill Belichick will be limited with what he can call.
Nobody would ever wish injury on Brissett, but it certainly would be interesting to see what the Patriots' offense looks like with Edelman running the show.