Morneau's career was derailed by a concussion he received while sliding into second base and colliding with an infielder. And much like with Jason Bay, he was never the same player after one.
And Sizemore had microfractures in his knee, much like a former Boston 2B, and was just never able to recover properly, despite trying for multiple seasons.
Just because it isn't the NFL doesn't mean it's not a physical game. You're largely highlighting players derailed by injury and labeling it PED usage. Hafner (who only layed 4 full seasons) and Hidalgo had a lot of injury problems as well. By this logic, Tony Congiliaro was clearly using PED's as well..