The Nationals placed Arano on the 15-day injured list Sunday with a right shoulder strain.
Arano's placement on the IL is retroactive to Friday, making him eligible for activation as soon as Sept. 17 if his shoulder issue proves to be a minor concern. The right-hander had been sharp over his last seven relief appearances prior to landing on the IL, as he struck out 11 over 6.2 scoreless innings while allowing eight baserunners.