Harper appeared to grimace after checking his swing in the bottom of the sixth inning, according to the Washington Times. The Nationals diagnosed him with a contusion and Harper will take medication to help with the discomfort.
Harper is likely to be out of the lineup for Thursday's game, but doesn't appear poised to miss much more time moving forward.
The injury may stem from a play on Tuesday's game. In the fifth inning of that contest, Harper crashed hard into a right field scoreboard while attempting to rob Tim Hudson of a home run. That play resulted in a large bruise down the left side of his body.
Harper, still just 20 years old, has been one of baseball's best hitters this season. He is currently tied for second in all of baseball with nine homeruns and is third with a 1.133 OPS.