When Joey Eischen went on the disabled list, the Nationals called up an outfielder to replace him on the roster. That left the team with 10 righthanded pitchers and no southpaws on the staff. This flies in the face of baseball convention, which dictates that 11 pitchers be on the roster and that a least one, preferably more, by lefties.
The first part, the one about the numbers, can be answered in two parts. First, Tony Armas is scheduled to come off of the disabled list early next week and there's no point is shuttling someone up from New Orleans to fill that 11th spot when he's just going to get sent down again. Second, if you don't have a lefty reliever, why hold a slot on the roster for one? You can choose between having another righty setup man, one who could be getting in some time on the mound in New Orleans or another bat on the bench. The bat on the bench seems to be the way that Frank Robinson has chosen to go.
Yes, it would be great if the team could make a phone call and either call up a lefty or trade for one for a reasonable price. But there are no lefthanders who are ready in the system and the trade premium for lefties is always high. It's better to have a mediocre righty going out there then a southpaw who is either not ready or highly substandard.
You can't put a 25-man roster together based on the players you wish you had, you have to work with what you've got. The current situation is a byproduct of the years of neglect of the minor league system. It's shameful that there is not a single lefthanded pitcher in the organization who can fill in the void.
But it is what it is.