Arano was placed on the 15-day injured list Monday with left knee inflammation.
Arano left Sunday's contest early after taking a spill trying to field a ground ball. He will now be away from the team for at least 15 days as he nurses the injury. Dee Strange-Gordon was activated off the paternity list Monday, filling Arano's roster spot, but the Nationals will likely look to call up an arm from Triple-A Rochester in the coming days.