Joel Hanrahan trade: Mark Melancon would be a good addition for Pirates, and ERA has nothing to do with it

The trade of Joel Hanrahan to the Boston Red Sox has been slightly unusual -- and annoying to write about -- because it has unfolded so slowly. As I write this late Saturday night, it's been happening for almost the entire day, and we still aren't totally sure who the players are. We do know the Red Sox will get Hanrahan and one other player, and the Pirates will get Jerry Sands, pitching prospect Stolmy Pimentel, and two other players.

Late Saturday night, it emerged that one of those two players might be reliever Mark Melancon. Pirates fans, already understandably upset at losing their star closer, quickly Googled Melancon and saw ... a 6.20 ERA?! What are the Pirates doing acquiring a reliever with a 6.20 ERA?!

These concerns are misplaced. While I'm not sold on the idea that the Hanrahan trade is a good one (it would be tough to be, since we still don't even know who's in it), acquiring Melancon is a much better idea than it initially appears.

ERA is a terrible metric to evaluate relief pitchers. It's not the most precise metric to evaluate starting pitchers, since it doesn't control differences in defense, ballpark, and luck, or the whims of official scorers (who decide on the difference between earned and unearned runs), or the rates at which a pitcher's fly balls turn into homers (which vacillate from year to year). For relievers, it's even worse. Relievers pitch short stints, and which earned runs they're charged with depend heavily on the pitchers who take the mound after them. If you're trying to figure out whether a reliever is good, don't look at ERA.

Hanrahan himself actually presents a great case study. When the Pirates traded for him in 2009, he had a 7.71 ERA. Oh noes! What was Neal Huntington doing? He doesn't know anything!

Actually, what Huntington saw was that the Nationals defense behind Hanrahan was atrocious, and Hanrahan was allowing a ridiculous, unsustainable .431 batting average on balls in play. Hanrahan had also posted fine peripherals, striking out 35 batters and walking 14 in 32.2 innings. He also threw 95 MPH darts and had a very good slider. All those factors, which undoubtedly played a much greater role in the Pirates' evaluation than ERA did, suggested Hanrahan could be a very good reliever. That's exactly what he became, and he did it immediately, posting a 1.72 ERA with the Pirates in 2009 after the trade.

That's an extreme example, and we shouldn't expect that kind of success with Melancon. But as with Hanrahan in 2009, ERA is about the least useful number you could pick from Melancon's stat line. In 45 innings with the Red Sox in 2012, he struck out 41 batters and walked 12, which are good numbers. Like Hanrahan, Melancon has good stuff -- his fastball averages 93-94 MPH (and was touching 97 toward the end of the season, when he was posting terrific numbers). Melancon also throws a cutter and a hard, diving curve that helps him get lots of ground ball outs -- he got ground balls on 50 percent of his balls in play last year, again a very good number.

The main problem Melancon had last season was that he allowed eight homers in 45 innings. If you're a pitcher, that's an annoying problem. But if you're a team acquiring a pitcher and hoping he'll bounce back, it's a great problem, because what percentage of a pitcher's fly balls leave the park is largely a function of luck. It's counterintuitive, but it's true. The statistic xFIP controls for a number of factors, one of which is the rate at which fly balls turn into home runs. It operates on the same scale as ERA. Melancon's xFIP last year was 3.45, a far cry from his 6.20 ERA. As long as Melancon racks up ground balls like he did last year, he should have a fairly low home run rate. If that's what happens next year, his ERA will likely come way down.

(Melancon's ERA also probably suffered from a tendency to allow strings of hits in games that were already blowouts -- he allowed more than half his runs for the season in four outings in which he entered with the Red Sox already way down. It's not clear that would be a pattern, and no reason it would have much effect on the Pirates' winning percentage even if it did. The Pirates would likely use Melancon in higher-leverage situations than the Red Sox have, anyway, meaning he wouldn't be pitching in blowouts very often.)

Again, this isn't to say that Mark Melancon is the next Joel Hanrahan. Guesses about the future are probabilistic, and there's always a chance a player acquired in a trade will be a flop. But Melancon's ERA last season tells us virtually nothing. In fact, the underlying numbers strongly suggest he's basically the same pitcher he was in 2011 with the Astros, when he saved 20 games and posted an ERA of 2.78. Melancon would be an excellent buy-low opportunity for the Bucs. I'm not thrilled about trading Hanrahan, either, but if Melancon is one of the four players in the deal, that's good news for Pirates fans. And ERA really has nothing to do with it.