All 30 MLB Teams' Biggest X-Factor as 2015 Season Gets Underway

From the cooperation of Masahiro Tanaka's right elbow to the rise of Mookie Betts, every team has an X-factor.

They are those impossible-to-predict but ridiculously important variables that will play a monster role in determining how the 2015 MLB season will shake out. The New York Yankees are far from the only squad with a serious injury concern that could make or break the campaign.

Aside from health problems, there's another theme that pops up again and again in the process of compiling the list of each club's biggest X-factor. From the Boston Red Sox to the Seattle Mariners to the Pittsburgh Pirates, there is a slew of playoff hopefuls who will be counting on young and promising players to step up in a major way this season.

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