4. Matt Cassel
There’s no doubt that Cassel had a bizarre career. He served for years as Tom Brady’s backup before finally taking over the position in 2008 while Brady recovered from injury. This convinced the Kansas City Chiefs that he was worth bringing to Missouri, and it all started so well.
Cassel had the season of his life as he helped the Chiefs to the playoffs and even made the Pro Bowl. However, it was all downhill from there as he went into severe decline. He served as a back-up at every other franchise he played for and never lived up to his full potential again.