According to a series of tweets from The Nats Blog, last year's top overall pick in the June draft hurt his ankle and was carted off the field.
A Nationals spokesman just emailed a statement to the media saying Harper was diagnosed with a sprained left ankle.
"Harper was removed from the field and immediately examined by the Nationals training staff," spokesman John Dever said in the email, adding that Harper is considered "day to day."
The 18-year-old Harper started the spring in the Nationals' major league camp because he was on the team's 40-man roster, but was optioned to the minor leagues March 12. Harper batted .389 (7-for-18) with three doubles and five RBIs in 13 Grapefruit League games.
He is expected to start the season at Single-A Hagerstown of the South Atlantic League.