Alex Smith
Smith developed into a decent quarterback after his nightmare in San Francisco. Kansas City saw him as a safe pair of hands before they drafted and started young gunslinger Patrick Mahomes. Then Smith moved to Washington but suffered a life-changing injury. He brutally broke his leg in a game against the Texans (via ESPN).

Then infection seeped into his bone and threatened to kill him. Doctors considered amputation because the situation was so severe. It took him almost two years to return to full health but nobody expected him to play again. He stormed back and claimed the Comeback Player of the Year award before retiring gracefully in 2020.