The 2011 version of Slaten is the posterchild for anyone who has ever argued that ERA is overvalued. He's not getting lefties out (.308/.333/.538 against in 2011) and should never face a righty at all (.375/.516/.625). Slaten's walk rate has skyrocketed to 6 per 9 innings, The sparkling 2.25 ERA is a product of having almost 95% of the runners he leaves on base stranded by the likes of Clippard, Storen and company. He's at his worst with runners on base (1.014 OPS against) and with 2 outs (1.208 OPS against). Basically, Slaten's 2010 magic has not been renewed this season.
Burnett is not nearly as bad as Slaten has been this year, but you can't tell by his 5.59 ERA. That being said, Burnett has still struggled mightily in 2011, with a K/9 drop from almost 9 to under 5. His control is better than ever, though, and his FIP stands at a solid 3.65. Burnett's contract makes him a better bet to stick around and work out his troubles in the majors, and for good reason in my opinion. He does have tons to work on, though, and in the meantime Riggleman should be reluctant to use him in key situations until his performance improves.
There are no perfect replacements for Slaten or Burnett. Rizzo undoubtedly wants to carry two LHP's in the bullpen (which would be great if either of them could actually get batters out), so that leaves us with a handful of options in AAA and AA:
Oliver Perez has a 0.75 ERA, including 6.75 K's per 9 and 1.50 BB's per 9 in 12 AA innings. He could surely use more seasoning in AA, but so far he has retired 12 of 14 lefties he has faced, with 0 walks and 4 strikeouts.
Atahualpa Severino could get the call if the Nats wait a week or two to make a move. The little lefty made his first appearance of the season on Saturday after starting the year on the DL. His solid 2010 season and place already on the 40-man roster helps his chances of getting called up sometime in June.
Ross Detwiler is another 40-man candidate who needs more time to make a case for the Majors this season. After a disappointing and injury-plagued 2010 season, Detwiler has stumbled out of the gate in 2011, with a 5.77 ERA. His performance is partly unlucky with a .400 BABIP and 63.3 LOB%, shown by his 3.43 FIP. Calling Detwiler up with the sole purpose of sticking him in the bullpen might be seen as a white flag by Rizzo, as trying to get anything useful out of the first draft pick in his tenure in the Nats' front office would be considered a last ditch effort at saving face. I have yet to give up on Detwiler's chances as a starting pitcher, however, and don't think this role would be good for him.
Tom Milone is a personal favorite, with solid performances in high A and AA ball over the past 2 seasons. Milone deserves a shot at the Majors, but I am not sure if this is the one that would be best for him; Milone is a starting pitcher and moving him to a LOOGY role could potentially mess with his development. He is definitely qualified, though, with 7.82 K/9 and 0.71 BB/9 against lefties this season in AAA.
Lee Hyde, a waiver claim from the Braves in March, has struggled with control so far this season (and in his entire career). With 7 BB/9 in AAA this season, Hyde is not going to sniff the Majors anytime soon. He could be useful down the road if the Nats' pitching coaches can fix his control issues, but they have not done so yet.
Cory VanAllen should get over the hump and make it to AAA this year, but I doubt that he will be in the Majors in 2011. His 5.59 BB/9 against lefties this year is discouraging, but his matching 5.59 H/9 and 13 K/9 are encouraging. 2011 is a make or break year for the 27 year old lefty, so he'll really need to improve his control to continue his development.
Chris Manno is the most intriguing player in the Nats' system if you ask me. With a 0.00 ERA, 13.5 K/9, 1.93 BB/9 and 3.1 H/9, he is absolutely dominating high A ball. The 2010 draft pick (also a '09 pick) has been tagged as a soft-tosser with a bizarre delivery, but you can't argue with these results. He's not going to be the first call-up but who knows, maybe Manno gets a September call-up this year if he continues this torrid pace.
My vote: dump Slaten for a RHP and keep Burnett up.