The injury appeared to come on a swing in the sixth inning, where the outfielder appeared to tweak something on a check swing. Harper stayed in to finish the at-bat with a groundout, but immediately talked to manager Davey Johnson and athletic trainer Lee Kuntz following the groundball.
When the Nationals took the field in the bottom of the sixth, Roger Bernadina had replaced Harper in the lineup and right field.
Harper also crashed hard into the outfield wall on Tuesday, which left a visible wound, although there is no information whether the two injuries are related.
The 20 year-old has swung the bat extremely well for Washington this season, hitting .337/.427/.705 with nine homers and 18 RBI's. Harper's hot start has been especially important for the Nationals this season, as Jayson Werth and Ryan Zimmerman are also battling injuries.