The Nationals officially called up Ross Detwiler to pitch for tonight against the Cubs, and in his place, they designated Collin Balester for assignment. But the Nationals are saying the move with Balester is more procedural than anything else, much like what they did with Matt Chico last season.
Update at 4:36 p.m.: Here's the skinny on what the Nationals are doing with Balester: They have to put him on optional waivers, instead of optioning him (or putting him on optional assignment) because his major league debut was more than three years ago. They ran into the same problem with Chico last year, and the action of designating Balester for assignment is mostly so the Nationals can have a roster spot open for tonight's game, before the 48 hours it would take for Balester to clear waivers. Essentially, he'll pass through waivers (players put on optional waivers are almost never claimed) and be back at Syracuse on Thursday.