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Starting rotation depth has been the foundation upon which the Mets have built their first NL East title in nine years. Between the budding young stars headlining the front of the rotation and the veterans Bartolo Colon and Jonathan Niese helping fill out the back of it, Sandy Alderson & Co. built a staff so deep that even Zack Wheeler’s torn UCL barely set them back this past spring.

The team has already begun transitioning Niese to the bullpen in preparation for the playoffs. If healthy, rookie Steven Matz will be the fourth playoff starter, but back stiffness has opened up the possibility that Colon could get the ball in a hypothetical Game Four. That the Mets have a roughly league average pitcher as Matz' understudy speaks to just how deep the rotation is.

While much of the organization’s success in cranking out quality arms can be attributed to organizational philosophy and player development, a lot of credit has gone to pitching coach Dan Warthen. The emergence of Jacob deGrom and Matt Harvey rest at least partially on Warthen's aid in developing their deadly 90 mph power sliders. The high-velocity, low-spin pitch was also passed on to bullpen stalwarts Jeurys Familia and Hansel Robles (and Jenrry Mejia, who has likely thrown his last pitch with the organization), and was dubbed “The Dan Warthen Slider” by Eno Sarris in an excellent FanGraphs piece in July.

It turns out that the two latest disciples of the Warthen slider are the two rookies that the Mets hope will complement deGrom and Harvey come playoff time.

Noah Syndergaard’s high-90s heat and “hook from hell” have been even better than advertised, and at the behest of Warthen he added a deadly two-seamer upon arriving in Queens. The Warthen slider is something that the blonde-locked fireballer has tinkered with at various points throughout the season. His use of the slider was inconsistent early in the season, as he threw 10 in a start against Atlanta on June 20th and another eight against Washington on July 22nd. Syndergaard told Sarris back in July that he had “played with it a little bit” and admitted that the pitch had affected the spin rate on his curveball.

At the time it seemed that Syndergaard didn’t feel completely comfortable using both pitches consistently. Toward the end of August there was evidence that suggested that he was dabbling with the pitch again, but that it might be bleeding into his curveball. However, since the calendar flipped to September, Syndergaard has clearly worked the distinct high-80s version of his slider back into his repertoire. This time, it looks like it’s sticking around for good.

During his start on September 12th, Syndergaard carved up the Braves lineup without enlisting the help of his curveball even once. He went to the slider 14 times during that start and the pitch averaged 88 mph. Here’s one of the two whiffs the slider generated on that night.

He brought the curveball back after the Atlanta start and has mixed in an assortment of high-80s sliders and low-80s deuces in his past two outings against the Yankees and Reds. Todd Frazier fell victim to both breaking pitches during a strikeout in the fourth inning. Syndergaard started him off with an 82 mph curveball out of the zone that Frazier chased for strike one

then finished him off later in the at bat with a sharp 88 mph slider that the Reds third baseman chased with all the grace of Bartolo Colon in the box.

We’re talking only 30 pitches, but Syndergaard’s slider has generated a 27 percent whiff rate during his three September starts. With the pitch now an apparent mainstay in his arsenal, he has five weapons. The two variations of his fastball both touch triple digits and allow him to constantly change eye levels. His high-80s changeup has served as a pitch to keep left-handers off balance and fill a velocity band between his heat and curveball. The slider now fills that band more regularly against same-sided hitters and gives him yet another potential swing-and-miss offering.

Steven Matz isn’t nearly as polished as Syndergaard but has shown flashes of brilliance during his brief tenure in the majors. Sarris mentioned at the time of his article that both Syndergaard and Matz had shown an interest in learning the slider but that the Mets didn’t want the latter experimenting with another pitch. Instead they wanted him to “focus on developing the curveball.” Two months later, the Mets felt comfortable enough with the left-hander’s progress to let him try out the slider. He debuted his new pitch on September 18th against the Yankees, mixing it in seven times. After the game Matz talked to reporters, including ESPN.com’s Adam Rubin, about his new pitch.

"I was working on it with Dan a little bit in the bullpen sessions. I told Trav before the game, 'If we get ahead of a hitter, just kind of show them it.' And it turned out to be working pretty well for me today. So we mixed it in a few more times."

While he didn’t throw the slider as often his next time out, he didn’t use it as a show-me pitch in the same way he did against the Yankees. Matz threw the slider only four times in his next start, but they all came against Cincinnati’s most dangerous left-handed hitters: Joey Votto and Jay Bruce. Matz opted for a 1-1 slider to get ahead of Votto in their first meeting

and then used back-to-back sliders against the slugging first baseman in his second at-bat.

At this point, Matz’s slider appears to lack the same type of late bite that Syndergaard’s does, but the pitch does give Matz another option against same-sided hitters. He had essentially been a two-pitch pitcher against lefties prior to adding the latest wrinkle to his repertoire. If he feels comfortable enough with the pitch to use it regularly in the playoffs, it will make him less predictable should he get the chance to square off against Adrian Gonzalez or Corey Seager in the NLDS.

With playoff baseball ready to invade Queens for the first time since 2006, the Mets pitching staff was already seen as the team’s primary strength. Matz insists that his back stiffness isn’t serious. The Mets hope the left-hander will check off every box during his outing this weekend so he can join deGrom, Harvey and Syndergaard to form a playoff quartet that boasts premium velocity like no other rotation in baseball. Under the watch of Warthen, the four of them now have something else they all share in common.