This is one of the biggest questions for the near-future Yankees: is the real Luis Severino the guy who in 2015 pitched like a #1-caliber starter at a nearly 6 WAR/yr clip, or the guy who in 2016 has a 7.19 ERA and can't get past the middle innings? The two main explanations aren't promising: (a) that he he needs a much better changeup ("he has only two good pitches" isn't a promising explanation for a starter's struggles); and (b) that his 4.37 FIP in 2015 hints that luck explains much of his performance last year.
Still, I'm flummoxed by seeing such terrible performance from a guy so great (a) just a year ago, (b) at such a young age, (c) as a rookie. On the one hand, maybe 11 strong starts isn't enough of a sample to mean anything. On the other hand, if you can badly overmatch big-league hitters over 11 starts at age 21, I'd think that means you're a quality pitcher, even if you later have growing pains or a sophomore slump. So to answer the question, "how much did Severino's 2015 really tell us," I ran a Baseball Reference Play Index search asking, "what past starters had seasons like Severino's 2015." Is a Severino-like stellar rookie performance for 60+ innings something a lot of stiffs luck into? Or do only guys who end up with strong careers do that well at age 21? I searched for anyone in the past 40 years with an under-3.00 rookie ERA, with at least 50 IP, at age 21 or younger.
Wow. Almost everyone on the list either would meet our wildest expectations for Severino (Fernandez, Hernandez, Bumgarner, Gooden, Strasburg) or at least had a #2-level starter career we'd happily take (Sanderson, Gullickson, Burns, and Wacha's up-and-down career so far). The only two flops are pretty non-analogous to Severino: Youmans, who actually did great the next season (3.7 WAR) after his strong partial rookie year, but then got derailed by drug problems; and Volstad, whose very low K rates made his rookie half-season look flukey from the start.
If this were a peer-reviewed academic paper, I'd have to run a few more searches, to avoid making Severino's comparables list too good with arbitrary ranges and endpoints. Maybe the 19-20 year-olds were more impressive, so I could hav excluded them; or maybe I could've excluded folks with 150+ IP, because a strong 150 IP is better evidence of lasting talent than a strong 60 IP.
But take it for what it's worth: yes, he lacks a good third pitch; and yes, his FIP hints that in 2015 he outperformed his core performance; but still, there's almost no precedent for a starter to perform like 2015 Severino yet then fail to have a good-to-great career. Or maybe it sounds better the opposite way: almost everyone who's pitched like 2015 Severino has gone on to a darn good career. So despite a season of awfulness from him, I'm still a Severino optimist.