Jason Jennings- Colorado Rockies
Rare is the occasion when a Rockies pitcher earns accolades for his accomplishments on the mound. But Jennings captured Rookie of the Year honors despite playing in baseball's most bountiful hitter's paradise. In his first game, he pitched a shutout and hit a home run--the only pitcher to do so. He never had another winning season after his 16-8 showing in 2002. In 2007, he underwent arm surgery that effectively ended his career.
Dontrelle Willis- Florida Marlins
With a flamboyant personality and dramatically high leg kick, Willis became the toast of the town when he helped the Marlins to a World series title in 2003. A resounding choice for Rookie of the Year, his career was star-crossed after that. Though he followed a sub-par 10-11 effort in '04 with a 22-win campaign in '05, his life began to unravel. In December of 2006, he was arrested for driving under the influence. In 2007, he began to lose the plate. By 2008, he was suffering from acute anxiety on the mound, which warranted a demotion to the minors. He made multiple unsuccessful comeback attempts.
Geovany Soto- Chicago Cubs
The Cubs rejoiced in 2008 when Soto landed among the top offensive catchers in the majors and received a Rookie of the Year nod. Though he continued to play well defensively after that, he lost his swing. In his sophomore campaign, he hit just .218. He bounced back in 2010 with a .280 mark at the plate, but the performance proved to be a fluke when he reached a personal low of .198 in 2012. He has served as a back-up catcher ever since.
Andrew Bailey- Oakland A's
According to Oakland's Moneyball philosophy, relievers are replaceable. Bailey seemed to be on his way to great things when he recorded 26 saves and captured Rookie of the Year honors in 2009. He saw increasingly less playing time over the next two seasons. Traded to Boston in 2012, he posted a 7.04 ERA in 19 appearances. He is currently property of the Yankees though he has spent most of the year in the minors.
Jose Fernandez- Florida Marlins
The right-handed Fernandez was an All-Star and Rookie of the Year in his 2013 debut. His career was derailed the following year when he sustained an elbow sprain and underwent Tommy John surgery. He returned to the majors in July of 2015, but complained of shoulder stiffness in August. Only time will tell if he can fully recover.
Wil Myers- Tampa Bay Rays
Myers made his debut in June of 2013 and was named Rookie of the Year on the strength of his .293 batting average and 36 extra-base hits in 88 games. He slumped to .222 in 2014 after sustaining a wrist injury in an outfield collision. Traded to Padres before the 2015 slate, his fortunes have not changed any. As of this writing, he has missed close to 100 games and spent a significant chunk of time in the minors.