The numbers behind Adam Warren's transformation

How did he become a key part of the Yankee pitching staff in 2014?To put it mildly, Adam Warren's first outing in a Yankee uniform did not go well. In June 2012, Warren was summoned from Triple-A to make an emergency start against the White Sox in the wake of injuries to both Andy Pettitte and CC Sabathia. The 2009 fourth-round pick couldn't make it out of the third inning, allowing eight hits – including two home runs – and six runs in a 14-7 loss.

Nearly three years removed from that disastrous debut, Warren has established himself as one of the team's most valuable middle relievers, and this spring has been given a chance to compete for a potential spot at the back of the rotation (and, at worst, will be the third-best arm in a deep bullpen).

Warren improved his performance dramatically across the board over the last two seasons, reducing his walks and homers allowed while increasing his strikeout rate to nearly one per inning. That resulted in near-half-run drop in his ERA to 2.97, which was backed up by an equally stellar FIP of 2.89 last year.

ADAM WARREN, 2013-2014

Season IP ERA AVG SLUG OPS K/9 BB/9 HR/9
2014 78.7 2.97 .219 .333 .615 8.7 2.7 0.5
2013 77.0 3.39 .268 .428 .766 7.5 3.5 1.2


How did Warren go from being the 12th or 13th guy in the bullpen in 2013, to earning high-leverage innings as a power reliever in 2014? Let's dig into the numbers to see how the former Tar Heel became one of Girardi's most trusted arms, and what it means for 2015.

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One of the biggest keys to Warren's improvement was his development against opposite-handed batters. Although he probably still walks too many guys, the boost in his strikeout rate and ability to limit hits has made him a formidable weapon against lefty-swinging batters.

ADAM WARREN vs LHB, 2013-2014

Season PA AVG OBP SLUG OPS K% BB% HR
2014 149 .178 .253 .271 .525 26.8% 9.4% 1
2013 173 .301 .370 .526 .896 17.3% 9.2% 7


The increase in his strikeout rate was due mostly to him missing more bats (swinging strike rate went from 10.8 percent in 2013 to 13.6 percent in 2014) and being able to put away those lefty hitters (47 percent of two-strike plate appearances ended in strikeout in 2014, compared to 34 percent in 2013). Those are two huge reasons why he became one of the Yankees' best middle relievers, and they're also skills that he's likely to carry forward into 2015.

The other key to his improvement against opposite-handed batters was the fact that he allowed just nine extra-base hits, including one homer, to the 149 lefties he faced in 2014; as a rookie, he gave up a whopping 20 extra-base hits and seven homers despite facing only 24 more lefties. Is that sustainable?

His miniscule homer-to-flyball rate vs lefties of 5.6 percent last season (it was 18.9 percent in 2013) screams regression, but a closer look at how he attacked those hitters and what they did on contact shows that Warren has a good chance of dominating left-handed batters again in 2015.

Nearly all of the damage that they did versus Warren two seasons ago came against his fastball, which was tagged for a ridiculous .722 slugging percentage by lefties in 2013. The problem was that he left far too many fastballs over the heart of the plate.

Adam  Warren Fastball Location vs LHB, 2013-14
Against opposite-handed batters, Warren reduced his percentage of fastballs thrown in the vertical middle-third of the strike zone (i.e., a batters's typical sweetspot) from 37 percent to 31 percent. As you can see in the heat map above, he traded those batting-practice meatballs for more high fastballs – and that improved location ultimately resulted in his rate of at-bats ending in a “hard-hit” ball being cut in half from 22 percent in 2013 to 11 percent in 2014.

Lefties simply couldn't square up on Warren's pitches last season, whether they were missing them completely or hitting them weakly to the infield for easy outs.

Based on the improvement in the location of his pitches and the decrease in his contact rate, Warren's transformation against opposite-handed batters over the last two seasons appears to be more real than fluke. He should be considered a viable weapon against both lefties and righties and a trusted member of Giradi's pitching staff for the 2015 season, either at the back end of the rotation or as a key middle reliever out of the bullpen.