The King is Dead, Long Live The King
In 1999, a completely unknown former grocery store bagger and Arena football star, Kurt Warner, replaced injured, high-priced quarterback Trent Green and led the St. Louis Rams to a Superbowl victory.
In 2002, a completely unknown former 6th round draft pick you had been cut three times, Marc Bulger, replaced injured, high-priced quarterback and two-time NFL MVP Kurt Warner, and led the Rams on a 5-game winning streak.
And it hasn't just been the Rams. In 2001, unknown Tom Brady replaced perennial pro bowler Drew Bledsoe and led the New England Patriots to a Superbowl victory over MVP Kurt Warner and the 14-2 Rams.
Two observations: First, this seems to happen a lot more in football than in baseball and basketball, despite baseball having the legend of Wally Pipp (who took a day off, was replaced by Lou Gehrig, and never played again). Second, doesn't this seem to happen a lot when you replace CEOs as well?