Tom Brady
Brady suffered a season-ending ACL injury in 2008 that cost the Patriots dearly. This gave Matt Cassel the keys to the quarterback engine out of nowhere. The backup guided New England to an 11-5 season and they narrowly missed the playoffs. Cassel fit into the system but Brady elevated it when he played.
That’s why his prolonged absence was a massive blow to the franchise. They had a strong roster but without their top signal-caller, they couldn’t reach their full potential. Meanwhile, Cassel tricked the world into thinking he was a decent NFL starter. Later, the Chiefs discovered this the hard way after they paid him (via Bleacher Report).