Hanrahan and Melancon may not be as different as you think.
The Boston Red Sox have made a series of moves this offseason in an attempt to pick themselves up from the train wreck that was the 2012 season. After signing Jonny Gomes, Mike Napoli, Shane Victorino, and Ryan Dempster in the past month, yesterday the Sox made an attempt to upgrade their bullpen by trading Mark Melancon and three prospects for Joel Hanrahan and a minor league infielder.
I'm no expert on the value of the minor league prospects involved, but the overall consensus seems to be that Brock Holt, the infielder that the Pirates gave up, is worth much less than Stolmy Pimentel, Ivan De Jesus, and Jerry Sands, whom the Pirates received in the trade. Consequently, the assumption is that Joel Hanrahan is a superior pitcher to Mark Melancon.
This assumption is an easy one to make, and is not necessarily incorrect. Just two seasons ago, Hanrahan saved 40 games in almost 70 innings, putting up a sparkling 1.83 ERA in the process. Then, last season, Hanrahan saved 36 with a 2.72 ERA. Melancon, on the other hand, all but fell apart in 2012 after a strong 2011 campaign in which he pitched 74 sub-3 ERA innings.
However, the peripheral numbers tell a slightly different story about the two relievers:
If we ignore ERA, the two pitchers no longer seem so dissimilar, do they? Hanrahan strikes out more batters, but saw a huge spike in walks and home runs, as well as a massive drop in ground balls, this past season. Melancon's peripherals didn't actually change significantly between '11 and '12, despite his inability to prevent runs. And in 2012, though Hanrahan had an ERA three and a half runs better than Melancon's, the latter beat out the former in FIP, SIERA, and kwERA.
Of course, we can't just take these numbers at face-value. Melancon only pitched 45 innings last season, so sample size caveats obviously apply. In addition, his utterly horrible results cannot be ignored - there was clearly something wrong with Melancon to get hit so hard, and we must take that into consideration when projecting his future performance.
However, we also cannot ignore Joel Hanrahan's serious regression in walks and ground balls in 2012. Despite the shiny ERA, Hanrahan struggled last season, and he cannot continue to produce positive results with those peripherals. Luckily, the walks and home runs will likely regress back to his career norm due to the small number of innings pitched, but that still leaves Hanrahan's true-talent ERA at 3.70 or so.
Can Melancon put up a 3.70 ERA next season? Of course he can. Really, any reliever can. There's so much noise involved in a 50-60 inning season that any attempt to predict reliever success is a fool's errand. But even when considering Melancon's true talent, it's not hard to make a case that he's a 3.50 ERA pitcher or better. In 157 career innings, Melancon has a 3.20 SIERA, largely due to his ability to get ground balls and limit walks. While he does not strike out as many batters as Hanrahan - an essential skill for relievers - he can still be an average to above-average relief pitcher.
Despite all this, it would be foolish to claim that Melancon is a better pitcher than Hanrahan given the former's utter failure last season and the latter's recent success and strikeout ability. However, it's not so crazy to think that Melancon is at least close in true talent to Hanrahan given his ability to limit walks and draw ground balls. The Red Sox upgraded their bullpen today, but given the unreliable nature of reliever performance and the two pitchers' similarity in peripherals, the significance of this upgrade may not be worth the pieces that they surrendered.