Nationals activate reliever Ryan Mattheus
Mattheus, who missed the last 58 games with a fractured right hand, has a 4.96 ERA in 14 relief appearances this season.
In six appearances during a rehab assignment, he had a 2.25 ERA pitching for the Gulf Coast League Nationals, Single-A Hagerstown, Double-A Harrisburg and Triple-A Syracuse.
Because teams are permitted to add a player for doubleheaders, the Nationals won't have to make a corresponding roster move until after Friday's games.
Washington manager Davey Johnson said Friday morning that reliever Drew Storen has the flu and was likely unavailable for the doubleheader.