Prior to pitching his final game of the 2015 season on September 5, there was a strong case to be made that Nathan Eovaldi was the Yankees' best starting pitcher. From June to September the Yankees went 12-5 in games started by Eovaldi, while he allowed an opponents' OPS of just .651 during that time span (compared to .822 opponents' OPS in April and May).
A key factor in Eovaldi's improved performance was the increased use of his splitter. The figure below from Brooks Baseball demonstrates the dramatic increase in the usage rate of Eovaldi's splitter as the 2015 season progressed. When the season began in April, Eovaldi threw his splitter just six percent of the time. By June that number had nearly quadrupled, with Eovaldi turning to his splitter for more than 21% of his pitches. From there Eovaldi's splitter use continued to climb in each subsequent month, and by the time his season ended, more than 1/3 of Eovaldi's pitches thrown (34%) were splitters.
Eovaldi's splitter made his pitch repertoire a tougher match for opposing hitters through two mechanisms.
First, Eovaldi's splitter is an extremely effective pitch. More than 29% of swings against Eovaldi's splitter in 2015 came up empty, yielding a swing and miss. And when hitters did make contact, it was rarely of very high quality. Only 14% of his splitters put into play were line drives, and 70% of splitters in play were on the ground. This incredibly high ground ball rate for Eovaldi's splitter was the driving force behind a better than seven to one ground ball to fly ball ratio against the splitter (7.42 GB/FB), and he did not yield a single home run against the splitter at any point in 2015. From a sabermetric perspective, more whiffs and ground balls make for highly effective pitches, and Eovaldi's splitter more than delivered on both of those fronts.
Second, his splitter has made his 97-98 mph fastball substantially harder to handle. After June, when Eovaldi began turning to his splitter more than 20% of the time, 66% of swings against Eovaldi's fourseam fastball culminated in a foul or a whiff, as compared to 56% in April and May when he utilized the splitter less frequently. In addition, post-June, Eovaldi's fourseamer yielded fewer line drives per ball in play (26% post-June versus 33% pre-June), fewer home runs per fly ball or line drive (4% post-June versus 7% pre-June), and more ground balls per fly ball (2.29 post-June versus 1.88 pre-June).
While Eovaldi's four-seam fastball velocity did trend slightly upwards as the season progressed, as well as exhibit slightly sharper horizontal movement, the most plausible explanation for his improved fastball outcomes is the increased usage of his splitter as a highly effective secondary pitch.
The knock on Eovaldi entering last season was the lack of a quality secondary offering to keep hitters from sitting on his high 90s fastball. That offering emerged for Eovaldi in June when he first began throwing his splitter with regularity, forcing hitters to think about a pitch in the lower half of the strike zone, and making his four-seam heat more effective in the middle to upper half of the zone.
Eovaldi is a key cog in the Yankees' rebuild while remaining competitive machine. He is young, inexpensive, and has demonstrated a great deal of arm talent early in his career. The development of his splitter is an important first step in his evolution from thrower to pitcher, and leaves Eovaldi poised for further progress in 2016 if he can remain healthy.