Submitted for your consideration: Grady Sizemore
Grady Sizemore was a center fielder from 2004-2015, but not really.
After an abbreviated rookie appearance in 2004, Sizemore became the starting center fielder for the Cleveland Indians in 2005. He did okay, becoming one of a handful of players to get 20 doubles, 10 triples, 20 homers and 20 stolen bases in the same season (others are folks like Henry Aaron, Willie Mays, Roberto Alomar and Lou Brock). Then he decided to do that again the next season. Over the next few years Sizemore would be considered one of the up and coming superstars in the league, averaging 6.2 bWAR over those first four magical years. Baseball Reference list his most similar player through his age 25 season as Barry Bonds. He’d racked up 25.8 bWAR through that 2008 season. He also won a Gold Glove that year, in case you were wondering about defense. It was a great start towards a Hall of Fame career.
And then. And then…
Elbow surgery. Hernia surgery. Left knee surgery. Another hernia surgery. Back surgery. Right knee surgery.
He'd only accumulate 2 bWAR the rest of his career. And since he had 2.2 bWAR in his age 26 season, that means he averaged below replacement level for the next six years, two of which were completely missed due to injury.
Of all sad words of tongue or pen, the saddest are these, ‘It might have been'.’ - John Greenelaf Whittler