Understanding Robert Gsellman, the Mets’ fifth starter

The Mets starting pitching situation — when healthy — can be perfectly described as an embarrassment of riches. After injuries to Jake deGrom and Matt Harvey, the Mets saw the emergence of two rookie hurlers in Seth Lugo and Robert Gsellman. Gsellman is seen as the presumptive favorite to take the fifth spot in New York’s rotation, yet there isn’t a great deal of chatter about him.

Gsellman wasn’t seen as an impact prospect entering 2016. The sinkerballer was tabbed as a fifth starter type by some and a reliever by others. Although he has high fastball velocity and consistently threw strikes , there were considerable questions about his secondary stuff. Overall, this is a pretty common profile that essentially hinges on his ability to start.

Statistically, Gsellman reinforced this profile. He crossed the 20 percent strikeout rate just once in his minor league career — in Low-A ball over 70 innings back in 2013.

Throughout his career, he generally sat around 18 percent, but with a four to seven percent walk rate. Gsellman’s knack for limiting free passes and using his sinker led him to some impressive seasons on the farm.

In his stint with the Mets, Gsellman showed he has quite a bit more in the tank than what he was projected for. Over 44 ²⁄₃ innings, he looked impressive posting a 2.42 ERA with a 22.7 percent strikeout rate and 8.1 percent walk rate, earning him a 2.67 FIP and 3.93 DRA. It’s easy to see that Gsellman was a major contributor to the Mets’ late-season run for the playoffs.

Using the new pitch tunnel metrics from Baseball Prospectus, we can take a look into how Gsellman’s offerings act coming off the mound. Pitch tunnels quantify the flight of two pitches and compare their differences throughout the whole flight, at the “tunnel-point” where the batter must decide to swing or not, and at the plate. This is useful for showing how Gsellman mixes his pitches and how his pitches interact off each other.

With a repertoire featuring six pitches, Gsellman primarily pitched off his sinker and cutter (the two pitches appear in over 80 percent of his tunnels). His incredibly effective sinker appears in 66.9 percent of them, which comes as no surprise. His sinker generated about 20 percent balls in play. Of those balls in play, 12.54 percent — or 62.5 percent — were ground balls. Whereas, his cutter was his swing and miss pitch with the highest whiff rate of 19.84 percent.

Looking at the granular tunneling data, his back-to-back sinker/split was his most used pair at 120 instances. Obviously, there were minimal differences with how the pitches compared. That said, when fishing for a groundball, it’s a sensible sequence. Gsellman’s next two instances, both at 43 pitches, are both ends of a cutter/sinker tunnel. Each has a flight time differential of over 20 milliseconds and between 8.8 and 7.5 inches of difference at the tunnel point. Gsellman’s cutter is approximately five miles-per-hour slower than his sinker so the break and speed differential plays a major role in these differentials. Yet, because they’re both pitches based off the fastball and can be thrown similarly they can deceive the hitter.

Gsellman isn’t just based on deception and sequencing. Gsellman saw bumps in velocity when getting to Queens, like some other Met hurlers. He topped out at 97 this season, which is pretty good for a “pitchability” guy. Gsellman has also entered the Dan Warthen Academy for Pitchers Who Don’t Throw Sliders Good and Want to Learn How to Throw Other Pitches Good, Too. This essentially boils down to the impressive results with Gsellman’s “cutter” — or “slutter” — as a swing and miss pitch.

Gsellman’s deceptive profile worked for him in the big leagues, but it has staying power. Despite the personally high walk rate — and a labrum tear that kept him from swinging — Gsellman performed well and positioned himself well enough to be the presumptive favorite for the final rotation spot in New York.

Anthony Rescan is a Featured Writer at Beyond the Box Score. You can follow him on Twitter at @AnthonyRescan.