The injury occurred in the eighth inning, according to reports. Harper, who plays for Harrisburg, hurt his hamstring while running from first to third on an Archie Gilbert double. Harper had to be helped off the field.
Harper will likely be placed on the seven-day disabled list, according to a baseball source.
Harper, the first overall pick in the 2010 First-Year Player Draft, is having a solid season, hitting a combined .297 with 17 home runs and 58 RBIs for Class A Hagerstown and Double-A Harrisburg.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.