This morning, Eric Longenhagen released his list of the Top 20 Mets prospects, giving kudos to an underrated system led Amed Rosario, who Eric noted could be the #1 prospect in the game when next year’s Top 100 rolls around. Right behind him, Eric went with Robert Gsellman, the Jacob Degrom lookalike who ended last year in Queens, helping pitch the Mets to a Wild Card berth. This was a bit of a deviation from where Gsellman has ranked on other lists, as Baseball America had him as the #7 prospect in the Mets system, while MLB.com had him at #5. Eric, though, put a 55 FV grade on him, the same rating as many of the best-known pitching prospects in the game, such as Michael Kopech, Tyler Glasnow, and Jose De Leon.

Given Gsellman’s track record, it’s not surprising that he would engender some pretty different points of view. A 13th round pick, he was a run-of-the-mill pitch-to-contact guy for most of his minor league career, running strikeout rates as low as 13% in Double-A as recently as 2015. As Eric noted, he mostly sat in the low-90s and pitched primarily off his fastball, so between not being much of a prospect when drafted to not having an out-pitch against low-level hitters, there weren’t a lot of reasons to get too excited about Gsellman’s future.

But then last year, the velocity began to tick up a bit, and he got his strikeout rates up closer to league average in both Double-A and Triple-A, which earned him enough notice to get a big league callup when the injury plague hit Queens and the team needed another starter to help down the stretch. And then, once he got to the big leagues, he was nothing short of spectacular, running a 2.42 ERA/2.63 FIP/3.38 xFIP in 45 innings of work. As a Major Leaguer, his fastball sat at 94, generated a bunch of groundballs, and his secondary stuff was good enough for him to post an above-average strikeout rate. Besides the fact that five of his seven starts came against the Braves and Phillies, there was basically nothing to argue with in his big league performances.

So, Gsellman is a great test case for how evaluators weight different types of information. On the one hand, we have four years of minor league data suggesting that he doesn’t get enough strikeouts to be a high-end big league pitcher, and you almost always want to go with four years of history over a month’s worth of data. On the other hand, not only is Gsellman’s performance in the majors the most recent data, but it also provides some pretty clear evidence that he’s not throwing the same stuff he was as a minor leaguer who didn’t miss bats. What you think of Gsellman’s future likely depends on how much importance you put on long-term track record versus how willing you are to believe that a small sample performance that doesn’t match the history suggests a change in skillsets.

Before PITCHF/x and Statcast, I’d probably be in the “small sample size” camp, pointing out that even including 2016’s improved minor league numbers, KATOH is still comparing him to guys like Aaron Cook, and suggesting we don’t get too excited about a small handful of starts against poor competition. But thankfully, with better data, I think we now better understand of the limits of yelling “small sample size” about everyone, and we have tools that allow us to more regularly identify guys whose track records lose relevance after a significant shift in skills. And when you read Eric’s write-up and look at what Gsellman threw in the big leagues, I think there’s enough evidence to suggest that not only is the optimism warranted, but that it’s possible that we’re still undervaluing him even now.

Let’s put the minor league numbers aside for a minute. Let’s just talk about raw stuff. In the big leagues, Gsellman primarily threw a sinker that averaged 94, ran his four-seam fastball up to the high-90s on occasion, and threw a couple of breaking balls and a change-up ranging from 82-89. Using the always-nifty pitch descriptions from Brooks Baseball, which turn the data into scouting-report style write-ups, this is what Gselllman’s stuff looked like in the majors last year.

His sinker generates an extremely high number of swings & misses compared to other pitchers’ sinkers, generates a very high amount of groundballs compared to other pitchers’ sinkers and has well above average velo. His fourseam fastball has slightly above average velo. His cutter generates an extremely high number of swings & misses compared to other pitchers’ cutters, has heavy sink and is a real worm killer that generates an extreme number of groundballs compared to other pitchers’ cutters. His curve is slightly harder than usual. His change is basically never swung at and missed compared to other pitchers’ changeups, is much firmer than usual and results in somewhat more flyballs compared to other pitchers’ changeups. His slider (take this with a grain of salt because he’s only thrown 18 of them in 2016) is thrown extremely hard, is an extreme flyball pitch compared to other pitchers’ sliders and has primarily 12-6 movement.

Despite the difference between calling his primary hard breaking ball a slider or a cutter, this matches up well with what Eric wrote, and the first sentence really emphasizes why Gsellman destroyed big league hitters last year. A 94 mph sinker that generates both an “extreme” number of swinging strikes and generates a “very high” number of groundballs is a huge weapon. For instance, here’s the leaderboard of swinging strike rate on sinkers from 2016.

Probably not a coincidence that there are three Mets on that list. Also not a coincidence; most of these guys are really good. Darvish, Arrieta, and Syndergaard are three of the game’s most elite pitchers, and Carrasco isn’t far behind. Guys who throw swing-and-miss sinkers have a great foundation, and Gsellman’s sinker put him in the top tier of bat-missing with the pitch.

But Gsellman might also be different from most of those guys, because his sinker also generated the 12th highest GB% of any sinker in MLB last year. In general, the guys who get high whiff rates on their sinker don’t also get high grounder rates. For instance, Velasquez had the highest whiff rate but the fourth-lowest grounder rate. Out of the 119 pitchers who threw at least 200 sinkers last year, Darvish ranked 79th in groundball rate, Arrieta ranked 65th, and Ray ranked 87th. Brandon Finnegan, the least encouraging comparison on the whiff rate list, ranked 110th.

The only other pitcher who ranked in the top 20 in grounder rate on his sinker and top 10 in whiff rate with the pitch was Carlos Carrasco, who ranked 19th in GB% with his sinker; Ventura was 22nd and Syndergaard was 24th, for the record.

So, yeah, Gsellman’s sinker. This looks like it might be a pretty special pitch. If all we knew about him was that he threw that, then there would be plenty of reason for optimism. But the good news doesn’t end there.

His minor league track record shows a guy who is able to pound the strike zone, and he did the same thing in the big leagues. This isn’t a Daniel Cabrera situation, where a guy with a great fastball is unable to throw strikes and puts himself in hitter’s counts where guys can sit on it and crush a predictable offering. As Eric notes, Gsellman’s entire thing as a minor leaguer was fastball command, only now he’s apparently commanding a sinker that might be among the best in the game.

The primary knock against Gsellman now is that the breaking balls still aren’t great, and as Eric notes, the change-up is kind of terrible. As, as a sinker-heavy right-hander who is probably going to move towards the slider as his primary breaking ball — he’s a Met, after all — there seems to be some risk that he might be vulnerable to left-handers. But then, there’s this.

Gsellman’s Splits, MLB 2016 Platoon BB% K% GB% xFIP wOBA Vs LHB 10% 28% 49% 3.13 0.267 Vs RHB 7% 19% 58% 3.60 0.292

Platoon splits are one of the things that can show up pretty quickly, especially if a guy has a limited repertoire that only works against one type of hitter; it’s almost impossible for a sinker/slider right-hander to accidentally strike out a bunch of lefties if he’s throwing from a low-arm slot. But that wasn’t Gsellman’s story, as he struck out a higher percentage of lefties than righties, and still got a bunch of grounders from them as well. His breaking stuff might not scare left-handers much, but it seems like the fastball is good enough to pitch off of against hitters from either side, and while we shouldn’t put any stock in the reverse-platoon aspect of things, it’s at least encouraging that lefties didn’t torch him in the big leagues.

So, based on what he threw in the majors last year, it seems difficult to cling to comparisons to guys like Aaron Cook or Mike Leake. Gsellman looks like he has one of the best sinkers in the game, sitting at 94 with movement and command. The secondary stuff isn’t great, but realistically, there doesn’t seem to be a huge difference between what Gsellman was throwing in the big leagues last year and what Aaron Sanchez rode to an All-Star appearance in Toronto. Sanchez throws a tick harder, but he’s an example of what a heavy sinking fastball that also misses bats can do, even for a pitcher that doesn’t do a lot of other stuff at a high level.

Without a true knockout breaking ball, he probably won’t run elite strikeout rates, but the reality is that a guy who throws strikes and gets groundballs doesn’t also need an elite strikeout rate to be a good pitcher. Even if he settles in as more of a Marcus Stroman or Sonny Gray, guys with roughly average strikeout rates, that’s still a high-end arm, and the profile doesn’t look too different from what Garrett Richards was earlier in his career, showing that these guys do add strikeouts as they develop sometimes.

Of course, not every velocity spike is long-lasting, and the story changes a bit if Gsellman goes back to throwing 92 instead of 94. Health certainly is no guarantee. But I think this might be one of the times where what a player was previously might be having too much of an impact on what we think he is now. Eric certainly wasn’t conservative in giving Gsellman a 55 FV grade and putting him in the upper tier of pitching prospects around the game, but I wonder if even that might be underselling the value of a big league ready arm who throws what Gsellman throws.

There just aren’t that many guys commanding 94 sinkers that miss bats and get ground balls. It’s easy to look at the rest of the stuff and say that it’s not special, but if he had another special pitch, he’d be the best pitching prospect alive. As is, he looks pretty good to me, even without a knockout breaking ball. And if he develops one, well, good luck National League.