For the most part, I generally neither like nor trust relief pitchers. Maybe it’s a function of the fact that their failures are generally so much more memorable and consequential than their successes—simply because it’s more painful to lose something you have (a late-inning lead, in this case) than to keep it—or maybe it’s that, like viola players or gym teachers, most do what they do because they couldn’t cut it at the next level up (starting games). It's also possible I see them all as tainted by association with obnoxious humans John Rocker, Billy Wagner, Jonathan Papelbon, and Jose Mesa.
Whatever the reason, it’s far more often that I unreasonably deplore bullpen arms (hiya, Chad Durbin) than appreciate them. Ryan Madson, who’s not just my all-time favorite reliever but probably in my top dozen or so all-time favorite Phillies, is one exception; Brad Lidge, who gave us all an unforgettably great life moment, is another. Jake Diekman is bidding to become a third.
A big part of it is that he's just cool to watch. Diekman is a tall, skinny guy who seems to be all arms and legs, utilizing an odd motion that, combined with his unusual velocity, has to be scary as hell to opposing lefty hitters. It would be unfair to shrug Diekman as nothing more than a lefty specialist, though crushing fellow southpaws is a huge part of his value: last season he turned them all into Jose Molina with a collective triple-slash against of .239/.277/.304. His career numbers against lefty hitters are even better: .202/.275/.242.
Of course, Diekman put up some fairly huge strikeout numbers last season against everybody: an even 100 Ks in 71 innings, or 12.7 per 9, third in the NL behind Kenley Jansen and Craig Kimbrel. He mostly utilizes a fastball that averages 96-97 miles per hour, throwing it more than 70 percent of the time last season; Diekman’s second pitch is a slider that’s particularly effective against left-handers.
Despite all this good stuff, Diekman’s top-line 2014 numbers came in well below what fellow bullpen mainstays Papelbon and Ken Giles did. His ERA of 3.80 was nothing special, and it’s not encouraging to see a guy allow more than 1.4 base runners per inning. It didn't help that he issued multiple walks in eight appearances, despite rarely working more than a single inning. Baseball-Reference actually had him below replacement level, at -0.1 WAR. That said, his FIP was 2.65, with a .367 BABIP hinting he pitched in some ill luck last season.
As for 2015 and beyond, the projection systems seem to view Diekman as a serviceable but less than spectacular relief option, above replacement level but worth only about half a win. On the other hand, he’s drawn at least one comparison to Andrew Miller, whom the Yankees made absurdly rich this offseason. With a higher-leverage role—the 7th inning for now, with straight setup and maybe even some save opportunities if and when Papelbon moves on—if Diekman has a Miller-like season, he could become a very compelling trade piece: he’s arbitration eligible for the first time in 2016, and has three additional seasons before reaching free agency. As David Murphy pointed out in a recent column, Diekman’s talent (and affordability), and the risk inherent to all pitchers but especially the max-effort types, all could create compelling grounds for the Phillies to deal him to a contender.