Jason Heyward was one of the brightest young stars in baseball, an elite defender and baserunner who also hit very well. Through his first six seasons with the Braves and Cardinals, he looked like a future Hall of Famer.
Then Heyward signed a monster deal with the Cubs, and things almost immediately went downhill. While his defense remained excellent, his offense cratered. His OPS over his first six seasons was 114; in the seven seasons since, it’s just 86. Heyward went from being 14% better than league average to 14% worse, and it happened almost overnight.