There are quite a few pitchers who are underperforming right now. Most of the pitchers considered underperforming are perennial All-Stars and Cy Young contenders who are not measuring up to their track record and lofty expectations (Justin Verlander, Dan Haren, Jake Peavy, Jon Lester, Javier Vazquez, Johan Santana). In other cases, the pitcher has an ERA that doesn't match their other numbers.
6.50 SO/9, sub-1.20 WHIP, 3.50+ ERA, 5+ starts
Ricky Nolasco 3-2, 3.66 ERA
Mat Latos 2-3, 4.19 ERA
That's it. There are 27 pitchers with 6.50 SO/9, sub-1.20 ERA and 5+ starts, but only 2 of them have an ERA over 3.50. Both Nolasco and Matos have allowed a lot of HR (7 apiece), especially compared to other pitchers on the list with 0 like Ubaldo Jimenez and Francisco Liriano.
Unfortunately, that problem doesn't always go away, but it can be overcome: as Curt Schilling got older, he improved his control, won more games, had a lower ERA, and still gave up a lot of home runs.
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Ricky Nolasco is seemingly on the verge of stardom, with his low WHIP and high SO/9 and SO/BB totals. The good news is that he's had excellent SO and BB totals for a couple of years, so that will probably continue. He's already dominant in many starts, and just needs to be more consistent. At some point, I think he'll figure it out and elevate himself into the upper-echelon of ML pitchers.
Mat Latos has been pretty good lately, especially in his last start (8 IP, 0 ER, 9 SO, 0 BB vs. Astros). His high ERA is mostly the result of a terrible start against the Marlins on April 26th (2.2 IP, 7 ER). He's only 22, so consistency could be a problem at first for him too, but his ERA (4.19) should improve because his other numbers are too good for it to continue to be so high.