During the first inning of the Nationals' game against the visiting San Francisco Giants, star outfielder Bryce Harper's leg buckled as he touched first base.
Harper struck first base awkwardly while running out a grounder and immediately fell to the ground in pain after appearing to slip on top of the bag.
Any significant injury to Harper could be devastating to the Nationals lineup, even with plenty of other talented players on the roster.
The 24-year-old Harper is a five-time All-Star and was the National League MVP in 2015. With Adam Eaton already out for the year with a torn ACL, the World Series contenders are getting thin in the outfield. He batted.330 that season with a career-best 42 home runs.
Andrew Stevenson took over in center field and Brian Goodwin moved to right field.