Andrew Luck
The Colts went 8-8 in 2015 as they finished second in the AFC South. They only won a game fewer than the Texans as they narrowly missed out on winning their division. The biggest issue was that they couldn’t keep Luck healthy. Their quarterback was a genuine elite talent but injuries took their toll.

He announced his retirement in 2019 at only 29. It was a major shock but concussions and a shoulder injury caused his departure. He wanted to retire with some working parts of his body. But it was a massive blow for the Colts who struggled to replace him (via The Guardian).