6. Adam Gase
Peyton Manning recommended his former offensive coordinator Gase to the New York Jets but it ended badly. He was one of the worst head coaches in their history. The self-professed quarterback guru also ruined Sam Darnold’s prospects of becoming a franchise QB. It was a complete shambles (via Fansided).

Gase coached Darnold badly and left him badly exposed throughout his tenure. It’s easy to forget that Darnold was very young when he joined the Jets. For context, Cincinnati Bengals passer Joe Burrow was older as a rookie than Darnold was in his first year. But Gase ruined his confidence and sent him on a downward spiral.