Matt Capps saw his ERA come back to Earth in May, after posting a 0.68 ERA at the end of April. He still pitched well, though, with an xFIP of 2.26 and 0 walks; his 5.73 May ERA can be explained by team-worst's in BABIP (.343) and LOB% (51.7%). He remains a good option with an xFIP of 3.36, second on the team. Expect his ERA to remain about the same as his 8% HR/FB and .335 BABIP regress to the mean.
Sean Burnett had a strong May, averaging over a strikeout per inning and posting an xFIP of 2.76. Burnett's high strikeout rate (9.2 K/9) should help him raise his LOB% (68.2%) or at least lead to getting more innings than Miguel Batista.
Which brings us to Miguel Batista who may make the Nationals regret demoting Brian Bruney very soon. His team-worst 0.78 K/BB is also bad enough for 3rd worst amongst MLB relievers, (John Lannan has the worst spot locked down amongst starters at 0.69 K/BB) and his ERA is set to explode any day now because that ridiculously low .208 just can't be sustained much longer. His 5.77 xFIP (which adjusts for his fairly unlucky 14.7% HR/FB) is 9th worst in the league.
The last bullpen highlight is reserved for Tyler Clippard who is striking everyone out (10.19 K/9). He's getting extremely lucky with his HR/FB (4.7%) and his LOB% (88.1%), but since he's throwing fast (91.8 mph on average versus 90.5 last year) it might be a good idea to keep giving him innings.
The Nationals bullpen may be getting a bit lucky right now: Doug Slaten, Tyler Walker, Capps, Burnett, and Clippard all have xFIPs above their career averages. If Soren and English can pitch well and the veterans can maintain their success, however, the bullpen should be all right.