Tony Conigliaro. August, 1967. Boston Red Sox outfielder.
Tony C. was one of the most promising players in many years (at that point he was the youngest man to ever reach 100 career home runs) when he was hit in the eye by a pitch. He missed the rest of the '67 season (in which the Red Sox went on to win the pennant against 100-1 odds). He was able to come back and play a little after that and had two decent seasons but never fully regained his prowess and sadly retired for good a few years later (due to eyesight issues as a result of being hit).
The Little League style of batting helmut was adopted by many major leaguers after that incident (prior to that the batting helmuts didn't include the ear/eye flap).
Even sadder, he died in 1990, at age 45.
To this day, I say that the 1967 Red Sox outfield was the best I've EVER seen. Carl Yastrzemski, Reggie Smith and Tony Conigliaro. It just doesn't get any better.
A promising career was cut short. Similarly, Dickie Thon was becoming a star when he was hit in the eye by a Mike Torrez pitch in '83 or '84. He eventually came back, but was never the same.
