The Nats don't have prospects to trade for relief pitching. Assume Giolito, Lopez, Difo, and Turner are untouchable...and Turner is legally untouchable until he become a member of the organization.
That leaves Ross, and we haven't seen enough of Ross to know much, and Tony Renda. I'm not sure a team would trade a major league pitcher for Renda.
Rizzo is testing minor league starters. That suggests no team wants to trade. Maybe Goodwin starts to hit and by the trade deadline the worst team in baseball (Oakland???) might be willing to offload a veteran relief pitcher. However, Billy Beane is the guy who traded away HRod.