Bryce Harper suffered a significant bone bruise in his left knee and is out indefinitely. Harper avoided any ligament or tendon damage.
The Nationals are optimistic that Harper will return at some point this season.
"We dodged a bullet here," said Mike Rizzo, adding that it was "pretty remarkable" that Harper avoided a more significant injury.
Harper suffered the injury when he slipped on a wet first base in the first inning of Saturday's 3-1 victory over the San Francisco Giants.