When it comes to pitching prospects, standard protocol is to let them start until they prove they cannot -- even more so when it comes to top pitching prospects.
It generally derives from a simple exercise in maximizing value. Namely, you want your best arms pitching the most innings. Therefore, whether you've got a major league pitching staff with holes to fill or you just want to position your prospects as trading chips, starters are appreciably more valuable than relievers.
Enter Domingo Tapia. Tapia -- one of the most talented members of the Mets' current crop of excellent young pitching prospects -- had a breakout campaign in 2012, posting a 2.68 FIP (the fifth-lowest mark of anyone in Low-A with at least 100 innings pitched) as a member of the Low-A Savannah rotation. In his first exposure to full-season baseball the 21-year old power-armed righty also posted a career-best 8.37 strikeouts per nine mark along with a very solid 2.65 walks per nine.
What's more, his outstanding 2.65 groundout-to-flyout rate was the third-best among all Low-A pitchers with 100 innings pitched in 2012 -- a direct result of his stellar mid-90's, power sinker. The pitch is legitimately one of the three best fastballs in the Mets entire farm system. Aside from premium velocity, it features excellent boring action, running in on the hands of righties and regularly turning bats into kindling. For some reference, it's akin to that of Jenrry Mejia (and that of newcomer Noah Syndergaard, for that matter).
The pitch is so effective that Tapia hasn't really needed much else. To this point, nobody has been able to consistently hit it (see, .227 opponent average) and what's more, he controls it quite well.
In terms of the rest of his repertoire, he does possess a promising, yet inconsistent, high-80's change-up -- that features the same good fade as the sinker -- as well as a much straighter four-seam fastball, that regularly reaches the triple-digits. He also throws a very raw slurvy pitch, the only rudiment of a breaking ball in his repertoire. Yet, most of his success has come from his special two-seam fastball. For that reason, some are wondering if, despite his youth, Tapia is already fated to relieve.
Typically, one would say that there's still plenty of time for a 21-year old with less than three full seasons of stateside experience under his belt to refine his secondary pitches. However, an important factor that I should mention is that the imposing, 6'4" righty throws from a very low, three-quarters arm slot (click images to embiggen):
(screengrab from MLB.com footage)
(screengrab from Mark Newman's footage)
As you can see, it's almost as fair to call it a high sidearm delivery as it is to say low three-quarters. Yet it generates tremendous arm action and is one of the main reasons why his sinker sinks so very much. However, unfortunately it also makes the development of an orthodox breaking pitch much more difficult than a more traditional three-quarters or overhand motion.
In a piece for Fangraphs, South Atlantic League analyst Mark Newman discussed this very conundrum. He stated:
"Tapia’s inability to throw a breaking pitch screams bullpen projection at the moment. This leaves Tapia in a tough spot as changing his arm slot may yield a decent breaking pitch, but take the bite out of his impressive sinker."
And so we have our central conflict: Do you tweak the arm angle in an attempt to maximize his long-term value -- while at the same time potentially hurting the sinker? Or do you let him do what he does best, throwing mostly sinkers with the understanding that he could very likely end up as a reliever in the long run?
It's a tough call. Which is why I don't really want to make it; I'll eat the cake I have, thank you very much. Call it a cop-out, but I'd be more apt to leave Tapia's delivery the way it is and have faith that he can develop a useable secondary pitch along the way. He could easily spend four more seasons in the minor leagues before getting 'too old'; that's more than enough time for me to hope on.
Additionally, if it were an impossible feat that'd be one thing. However, all you have to do is look around the league at some current sinkerballers* to see that it is not. Here are arm angles from some of the league's most prominent sinkerballing starting pitchers:
Obviously sinkerballers come in all different degrees of arm angle; however many of them do share that same low release point. But the key point to keep in mind here is that each of these five pitchers has produced a positive pitch value (as defined by Fangraphs) on a curveball or a slider at least once -- but in most cases numerous times -- in their career.
As that relates to projecting Tapia's future, it's still unclear whether or not he'll be able to adapt to the rigors of starting at the highest levels. However, what is clear is that it's too early to say that he can't, especially based on his delivery.
*defined quantitatively as producing over 50% of their outs via groundballs and qualitatively as having a reputation for throwing a "sinking fastball"