It is true that his other stats aren't as good: his FIP is 3.85, his xFIP stands at 4.11. For the sabermetrically-minded, his K/BB ratio of 67/41 in 96 innings doesn't exactly inspire confidence; he's keeping the ERA so low because he's given up just 66 hits and a .199 average.
Pure luck? Skill? He's very much come out of nowhere, or so it seems anyway: Locke actually has an interesting track record as a prospect and it's not like he's never pitched well before. Let's take a look.
Locke was drafted by the Atlanta Braves in the second round in 2006, from high school in North Conway, New Hampshire. He posted a 4.22 ERA but with a sharp 38/5 K/BB ratio in 32 innings in the Gulf Coast League after signing. He was supposed to be rather raw when drafted, but his command turned out to be much better than expected. I gave him a Grade C+ entering 2007.
He followed his debut up with a 2.66 ERA with an outstanding 74/8 K/BB ratio in 61 innings for Danville in the Appalachian League, drawing further praise for his pitching feel. At this point I had him rated as a Grade B-. 2008 saw his ERA spike to 4.06 due to 150 hits in 139 innings for Low-A Rome, but he posted a solid 113/38 K/BB ratio and still drew positive comment for his pitchability. I still rated him as a B- entering '09.
He had some problems adapting to High-A, resulting in a 5.52 ERA in 10 starts for Myrtle Beach. Traded to the Pirates in the Nate McLouth deal, he posted a 4.08 ERA in 17 starts with a 56/18 K/BB in 82 innings after the trade for Bradenton, but allowed 98 hits. Although he still threw strikes, his command within the strike zone reportedly took a step backwards and his velocity was inconsistent. I lowered him to a Grade C entering 2010.
Locke rebounded in '10, posting a combined 3.56 ERA with a 139/26 K/BB in 144 innings between High-A Bradenton and Double-A Altoona. He got back into the low 90s and showed improved command, so he went back up to a Grade B- entering '11.
He went 8-10 with a 3.70 ERA between Altoona and Triple-A Indianapolis, with a 139/55 K/BB ratio and 143 hits allowed. He also made his major league debut with four starts, though he went 0-3, 6.48 with an ugly 5/10 K/BB in 17 innings. His velocity was again inconsistent in '11 and I lowered him slightly to a Grade C+ entering last season, noting that he needed another 10-15 starts in Triple-A, but still had a shot at being a fourth starter.
2012 was a very good year: 10-5, 2.48 in 24 starts for Indianapolis with a 131/43 K/BB in 142 innings. He made six more starts for the Pirates and got tagged with a 5.50 ERA, but his 34/11 K/BB in 34 innings was good and he showed enough promise to remain in the rotation mix for 2013. He earned the spot this spring, and as noted he's been very effective.
As his track record shows, Locke's velocity can be inconsistent. His fastball ranges anywhere between 87 and 94 MPH, averaging right at 90 MPH. He mixes in a curveball and changeup, and while he's not generally overpowering, he has enough stuff to succeed when his command is on. Carson Cistulli had a conversation with Locke in late May, who points to greater confidence as a contributing factor to his success this season.
He's also had some good luck and a 2.06 ERA seems unsustainable over the course of a full campaign, given his component ratios. However, if you look at Locke's entire major league career so far, 2013 plus the cups of coffee in '11 and '12, you find a 3.36 ERA in 147 big league innings with a 106/62 K/BB, 123 hits allowed, ERA+ 109, FIP 4.31.
Such numbers are not out of context with his minor league career at all. He is also quite durable, pitching as a full-time starter since June 2007 without getting hurt. That has value.
My overall take: Locke is above his head with the 2.06 ERA and we shouldn't expect that going forward. However, looking at his entire body of work, he has the ability to be a slightly better-than-average starter over the long haul, capable of giving you 180+ innings and remaining in the rotation for some time, a classic number four starter profile.
You don't have to be Clayton Kershaw or Cole Hamels or CC Sabathia to count as a success story, or be valuable for your team.