Nats have a bit of a 40 man roster squeeze, especially on the pitching staff.
They have Doo, Harris, Romero, Ross, Anibal, and Mason Thompson on the 60 day IL. Ross is throwing simulated games, threw 33 pitches, and hitting 95, so it looks like he should be back once he builds up innings and pitches. 4-6 weeks? Doo might be back close to the all star break assuming he starts throwing middle of next month. Thompson, Sanchez, Harris, and Seth Romero are not throwing.
Most to the minor league pitchers on the 40 man are recent waiver pickups or guys we recently traded for. Evans Lee is the exception. Perez and Abbott are 24 and 26, Machado 29, Clay is Claytus the Lefty, and then there's Carrillo (young, traded for, and hurt in Harrisburg).