I don't know a ton about hockey either, but apparently goalie is more analogous to kicker or punter than "regular" positions in football.

Even so, in the case of a kicker/punter injury a NFL team wouldn't call someone out of the stands to fill in - they'd sign someone out of free agency to play the next week and would adjust their strategy accordingly during the current game.

It's really cool that pro hockey has a unique system in place for edge case situations like this.