The Nationals formally recalled Ross Detwiler from Class AAA Syracuse, putting the left-hander in line to start tonight's game against the Cubs.
The club's corresponding move, however, came as a surprise: Reliever Collin Balester was designated for assignment, removing him from the organization's 40-man roster at least temporarily.
Balester is in his final option year, so the Nationals could have sent him to Syracuse without risk of losing him. However, by designating him, the club clears a spot on the 40-man which could be used to add another player.
A Nationals official said the club expects Balester to remain in the organization, though, and should be back on the 40-man roster within two days.
UPDATE AT 4:43 P.M. -- To clear up some confusion here (both yours and mine), this was a mostly procedural move. It turns out the Nationals can't option Balester to Syracuse because more than three years have passed since he made his big-league debut. Though technically he's been DFA'd, there's no risk of the Nats losing him. He'll remain in the organization.