The previously undisclosed ailment could be the reason that the Washington Nationals star is having a rough follow-up to his unanimous MVP campaign of 2015.

What’s wrong with Bryce Harper? One of the more frequently asked and vexing questions of this season may have its answer in a previously undisclosed injury. The Washington Nationals rightfielder and 2015 National League MVP has been playing through a right shoulder injury for the past two months, according to a source close to the team.

The injury affects the area at the top and back of his shoulder and at the base of his neck. Harper has been receiving treatment for the injury, the source said, including cupping therapy and Active Release Technique. Harper is hitting .233 with 20 home runs and 57 RBIs. Last year he hit .330 with 42 homers and 99 RBIs.

The injury has been particularly problematic for Harper when it comes to pulling his bottom hand through his swing, the key to creating loft and power. Harper, 23, has the highest contact rate and lowest strikeout rate of his five-year career, but his percentage of soft-hit balls is a career worst. His batting average on balls in play (.237) is the fourth-worst among qualifiers, trailing only the White Sox' Todd Frazier, the Indians' Carlos Santana and the Rangers' Prince Fielder, the latter of whom ended his career this week because of repeated neck problems.

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The exact cause of Harper's injury is unknown, though he is believed to have aggravated the shoulder on a head-first slide two months ago. Over his past 48 games Harper is hitting .208/.335/.353, with a .220 average on balls in play. Harper does not have a home run in his past 47 at-bats while hitting .128. He has played in 105 of the Nationals’ 113 games. Washington leads the NL East by 7 1/2 games, the second-biggest advantage in any of baseball's six divisions.