The 22-year-old left-handed reliever has a 6.75 ERA since the all-star break.
More additions to the newly expanded roster are expected.
The left-handed reliever was sent to the minors in a push for space and is expected back on Sept. 1.
The reliever tossed a scoreless inning in his first appearance since returning from the minors.
Less than a year after becoming a reliever, Ian Krol has helped fix the Nationals bullpen.
The 22-year-old rookie, nearly two weeks into his promotion from Class AA Harrisburg, has allowed only one hit.
In recent weeks, three pitchers have debuted for the Nats. A fourth -- one with big league experience -- is also rejoining the squad.