3. Rusney Castillo
The Boston Red Sox hoped that Rusney Castillo would be another Yasiel Puig or Yoenis Cespedes. He was not even close.
He was reportedly rusty as he had not played baseball in nearly two years when he made his major league debut. Castillo showed some promise in that brief stint in the majors in 2014, notching 12 hits, including two homers, in his 40 plate appearances. He was listed as the 21st best prospect in the game by Baseball America heading into the 2015 season as the sky appeared to be the limit.
Instead, those ten games were the highlight of his Red Sox tenure. He was mediocre in 2015, posting a 72 OPS+ in 289 plate appearances, and then received just eight more plate appearances during his time in the organization. While he was solid in Triple-A, the Red Sox outrighted him from the 40 man roster and never brought him back, making his six year, $72.5 million contract even more of a disaster.
The Red Sox were finally able to move on from Castillo after the 2020 season. He signed in Japan for 2021, appearing in a grand total of 50 games with three extra base hits. His time in the Mexican Winter League was better, but none of his 17 hits went for extra bases.
The Boston Red Sox hoped that they would have a star for their outfield in Rusney Castillo. Instead, they essentially lit $72.5 million on fire.