Every April, a few players get off to a bad start to the season. Last year, Derek Jeter (.250/.311/.272), Dan Uggla (.194/.250/.380) and Adam Jones (.218/.260/.414) struggled in the first month of the season, but because of their strong track records, they were able to play the "it's early" card, and their numbers improved significantly as the season wore on.
In general, you don't want to evaluate a player based on the small sample of less than 100 plate appearances, and overreacting to an early slump is not a good idea.
However, for these five players, if they don't come out of the gates strong to begin the year, they might not get any more chances to turn it around.
Hopes were high for Niemann when the Rays took him with the fourth overall pick in the 2004 draft, but between injuries and mediocre performances, he's never lived up to his draft position. Now 29 years old, Niemann had to battle Wade Davis for the final spot in the rotation, and while he was proclaimed the winner of the spring training faceoff, the Rays won't hesitate to go back to Davis if Niemann doesn't pitch well.
Few teams have a ready-made replacement sitting in the bullpen looking over their fifth starter's shoulder, but Davis' presence is intensified by the four-year contract he signed with the team last year. The Rays have a lot of incentive to get value from Davis, and a slow start from Niemann could easily push the team into flipping the two and sticking Niemann back in the bullpen.