According to Robert Flores of MLB Network, Nationals general manager Mike Rizzo called the setback a bone bruise while noting Harper avoided ligament and tendon damage in the knee.
Jamal Collier of MLB.com shared the general manager's comments:
Jamal Collier @JamalCollier
Here's Rizzo giving the diagnosis on Harper: https://t.co/ARYjylj8So2017-8-13 15:27:22
Harper's injury came when he tripped on first base and fell to the ground in obvious pain. He was helped off the field.
Dan Kolko of MASN said the Nationals didn't put an official timetable on the slugger's return but granted it "doesn't seem like a season-ender."
Kolko noted after the game Harper did not have a brace or even a significant limp but did wrap his leg below the knee after what appeared to be a major injury. Kolko also added Michael A. Taylor will come off the disabled list to take Harper's spot on the roster while the latter heads to the DL.
Washington is a head-turning 14.5 games ahead of the Miami Marlins in the National League East and appears comfortably on its way to the postseason. It has the cushion in the standings to survive this setback, and it is far more important from its perspective the star slugger returns to the field by October for potential postseason showdowns with the Los Angeles Dodgers and other National League contenders.
Harper was making an MVP case for himself before the injury with a slash line of .326/.419/.614 to go with 29 home runs and 87 RBI.
The Nationals will rely on other powerful bats in their lineup, such as Daniel Murphy and Anthony Rendon, until he is ready to return.