"[The bullpen session] was great. Better than the first one," Mattheus said. "The ball is coming out real good. I had no issue with the arm, which is the biggest thing. ... I let it go a little bit more and it felt game like. My stuff was really good."
Mattheus will have another bullpen session on Wednesday. If everything goes well, Mattheus hopes to have a simulated game when the team returns to Nationals Park this weekend.
Mattheus is expected to be activated by Sept. 20, when the Nationals play a doubleheader against the Phillies in Philadelphia.
Mattheus has a become a reliable reliever for manager Davey Johnson. In 32 games, Mattheus has a 2.40 ERA.