Polanco was a highly touted prospect prior to his debut last season, but then struggled and hit .235 in 89 games. However, Polanco now knows what it takes to succeed at the Major League level. and it's not like his natural talent disappeared. If Polanco can perform like he's capable of, it will be a big addition to the Pirates lineup.
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After dominating in the minors, Nelson wasn't exactly brilliant in the Majors last season. However, he wasn't bad either. The Brewers traded Yovani Gallardo over the offseason, creating an opening in the rotation for Nelson. The 25-year-old will get his chance to shine this season, and his success will be vital for the Brewers.
After a breakout rookie season, the Reds are hoping Hamilton avoids the infamous sophomore slump. Hamilton, once again, will pace the Reds lineup, and if he's able to increase his on-base percentage, it will be a good sign for the Reds and will likely lead to a lot of runs.
Bryant's reassignment to Triple-A garnered a lot of attention, but unless something drastic happens, Bryant will be called up fairly soon. The 23-year-old slugger was brilliant in spring training, and if he continues to hit once he's called up, it could be a special season in Chicago.
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The 2015 season will be Martinez's first as a starter. Martinez has shown tremendous ability in the past but has not been able to put it all together on a regular basis. The 23-year-old had a 2.86 ERA in spring training and may prove to be a very valuable cog in the St. Louis rotation.