David Robertson, undervalued Yankee asset?

After a miserable start to the year, Robertson has pitched his way to a decent season. Here are his 2010 splits:

Robertson is still prone to walking too many batters, but after a catastrophic April and a shaky May he has rounded into roughly the same form we saw last season. On the year he has pitched to a 3.68 ERA and a 1.46 WHIP. Both are worse than 2009′s numbers of 3.30 and 1.35, respectively, but in the world of “what have you done for me lately” Yankee baseball, it’s the June to present numbers that matter, and those numbers have been spectacular.

Is Robertson the second-best reliever in the Yankee bullpen? Let’s take a look:

All the data in this table are from Baseball Reference. aLI stands for average Leverage Index, a statistic that tries to measure the likelihood of a lead change in a given inning or situation. 1.00 means average. Less than 1.00 means less likely. Above 1.00 means more likely.

A few things surprised me about this data. First, Kerry Wood has been great for the Yankees, but his use in low leverage situations and his elevated WHIP — relative only to his ERA — mean that he’s been getting lucky. (He is also, clearly, the second-best reliever in pinstripes, but he may be a rental.)

Second, Joba Chamberlain hasn’t pitched as badly, in terms of WHIP, as his ERA would suggest. His perplexing season continues. Boone Logan, meanwhile, has not been as good as his ERA would suggest. He’s allowed the exact same number of base runners as Joba, except he is used in much less important situations, no doubt helping him a bit.

Finally, I was surprised to see that Robertson is not the reliever with the second-highest aLI on the Yankees. The original inspiration for this post was a belief that Robertson is usually the reliever Girardi turns to first, often to clean up a mess, when runners are on base. This hypothesis didn’t bear out. That kind of usage would suggest a higher aLI.

However, Joba and Robertson are probably heading in opposite directions in their average leverage. Joba’s numbers may be inflated from his early season appearances when he was the key set up guy (a job that now belongs to Kerry Wood). Robertson, meanwhile, was probably used in lower leverage situations in May and June, after his atrocious start, and has since been called on in tougher spots.

With Kerry Wood in the bullpen Robertson is probably not the Yankees’ second-best reliever, but he’s in the conversation. From June onwards his ERA is 2.11 and his WHIP is 1.29 in 33.4 innings of work. If the 25 year-old Robertson can avoid another terrible start, he may emerge as one of the gems of the Yankee bullpen in 2011.