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Reds SP Aroldis Chapman
Chicago Cubs: CF Brett Jackson
Jackson entered last season as the No. 32 prospect in baseball according to Baseball America, and he made his big league debut on August 5.
Though he is a legitimate 20/20 threat right now, he has to cut down his strikeouts if he wants to be a big league regular. He struck out 158 times in 407 at-bats at Triple-A Iowa, then whiffed a whopping 59 times in just 120 at-bats in Chicago.
Cincinnati Reds: SP Aroldis Chapman
After he posted video game numbers in the closer's role last season (38 SV, 1.51 ERA, 122 K, 71.2 IP), the Reds have opted to move the flame-throwing Chapman to the starting rotation this coming season.
Jonathan Broxton was re-signed to close, and Mike Leake is still around as a fallback starter option should the experiment fail, so the Reds are covered either way. Still, the transition is one of the biggest storylines of spring training.
Milwaukee Brewers: RP John Axford
With the top offense in the NL last season returning intact and a solid group of young starters filling out the starting rotation, the Brewers' biggest concern will once again be the bullpen.
Mike Gonzalez, Burke Badenhop and Tom Gorzelanny were added to bolster the 'pen, but Axford will once again be the anchor. After winning Rolaids Relief Man honors in 2011, he struggled last season and even lost his job at one point. He'll need to be his dominant self once again, or he may lose his job for good this time around.
Pittsburgh Pirates: OF Jose Tabata
Tabata burst onto the scene in 2010, hitting .299 with 19 steals to finish eighth in NL Rookie of the Year voting.
The following August, the Pirates signed him to a six-year, $15 million deal with options for 2017-19, as he was expected to be a core member of the young team. Instead, he hit just .243 last season and was demoted midyear. Now, he opens the season as a fourth outfielder, and he'll need to play his way back into everyday at-bats.
St. Louis Cardinals: CF John Jay
It's no fault of Jay's that he finds himself on the hot seat, as he hit .305/.373/.400 as the everyday center fielder last season.
However, top prospect Oscar Taveras is knocking on the door, and he could force Jay to a fourth outfielder role by midseason. With Carlos Beltran's injury history and the fact that Beltran is a free agent at the end of the year, Jay still has a key role on the Cardinals moving forward, but he may have to take a step back in 2013.