According to an eyewitness, William Yoder, Harper hit a groundball to short and didn't appear hurt until his foot hit the first-base bag.
Harper was carted off the field and immediately examined by the Nationals' training staff. He is currently listed as day-to-day.
Once Harper went down on the ground, Yoder -- a Washington, D.C., native -- moved closer toward first base to see how Harper was doing.
"Everyone was kind of shocked," Yoder said. "It looked like he was in serious pain. His eyes were closed, so it looked like he was upset. He didn't say anything [as he was carted off]."
Vowing that he wanted to be on the 25-man roster on Opening Day, Harper -- the first overall pick in the 2010 First-Year Player Draft -- played like it. He appeared in 13 Spring Training games and went 7-for-18 (.389) with five RBIs before being optioned on March 12 to Class A Hagerstown, where he is expected to begin the season.
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.