After the piece about bullpen mismanagement sparked a commentary about Jeremy Horst and lefty/righty splits and relievers I thought I would write a followup piece on the anatomy of a bullpen and the type of pitchers you find there.
Fastball: This includes straight 4-seamers, 2-seamers (downward and armside movement), sinkers (think 2-seamers with much more downward movement), and cutters (sharp slider break at a higher velocity and with smaller overall movement). The fastball isn't all about velocity (though it helps) there is also movement and command that matter. Major league hitters can hit fastballs of any velocity it they are straight so movement is a big deal and you need to be able to throw it for strikes, and if you can control where it is a strike even better.
Breaking Ball: In this day and age breaking balls tend to be limited to curveballs and sliders, though occasionally a pitcher will use a cutter as a breaking ball (also can be used as a changeup). Both sliders and curveballs will break away from a same handed batter making them a weapon against those hitters. The key to a breaking ball is the movement, ideally it has two plane break meaning it is moving horizontally and vertically (sliders that just break horizontally will be crushed repeatedly by opposite handed hitters). Also important is that you look the same throwing it as you do throwing your fastball, late and sharp movement is also encouraged.
Changeup: The changeup is a pitch that looks like a fastball but isn't. This includes every grip and movement of changeup as well as splitters (split finger fastball) and sometimes cutters. The goal of the changeup is to fool a hitter who thinks it is a fastball, and they are at their most effective against opposite handed hitters. Depending on the type of changeupit movement will be different, a splitter will have big downward movement as will a split finger changeup. A circle change (Hamel's changeup) will have slight sharp down and armside movement. Some changeups have no movement but the velocity difference to the fastball can still fool hitters.
How you end up in a bullpen:
We often generalize relievers as failed starters. Often this is the case, but in reality there is a different skill set needed to relieve than start. Here are some specific problems that could lead to a pitcher being made a reliever.
Health Concerns: Pitchers are injury prone by the nature of their craft (throwing a ball overhand is one of the most unnatural movements we do) but some pitchers are more injury prone than others. This can be due to preexisting wear and tear on their bodies, violent mechanics that will eventually lead to injury, or anything else that will contribute to a team being concerned there is only a limited number of pitches in their arm.
Large Platoon Splits: Part of being a starter is that you have to go through a whole lineup multiple times and you are on the schedule for a day. If you can't handle a lineup of opposite handed batters you aren't going to succeed in the rotation. This can be due to delivery (the lower the arm angle the better the view opposite handed batters have of the ball out of the hand and the more deception you have against same handed batters), lack of a pitch we will get to that later, or another point of deception that makes you susceptible to opposite handed batters.
Lack of Stuff: You can read this as lack of fastball velocity. Often pitchers will get 1-3 mph boost in fastball velocity out of the bullpen which can make a fringy fastball usable enough to set up better secondary pitches. Even if your secondary pitches are elite you need a fastball to keep hitters honest.
Lack of a Third Pitch: To be a successful starter you need 3 pitches (there are some exceptions but they are few and far between). This has to do with going through a lineup multiple times giving hitters different looks as well as not have huge platoon splits due to the lack of breaking ball or changeup. In the bullpen a pitcher often shortens to two pitches and their is less of a worry about that development.
Mechanics/Control: These two are often linked because the reason that the control is bad is often that the mechanics fall apart over time. In the bullpen you can pull a guy quickly if they fall apart and they need to get through less batters before they fall to pieces.
Need: Big league club need can sometimes cause a starter to be moved up in a high leverage bullpen role. Sometimes they can be transitioned out of the role (David Price, Adam Wainwright) and sometimes they dominate and you can never get them back out (Papelbon, Feliz, Chapman).
Lack of Size: This may not be the best thing the industry does but there is a bias against short pitchers. The first reason is there is a fear they won't hold up under heavy workloads. The second is the lack of plane on their fastball. A 6' 6" pitcher is going to get a lot more downward angle on their fastball than a 5' 10" pitcher, and that difference can make the larger pitcher more difficult to hit and generate more ground balls, while making the smaller pitcher more home run prone.
Let me start by saying I think the Save and Hold stats have done more to hurt the game than any other stat is baseball; wins, RBI, and runs are mostly harmless, whereas the save not only determines bullpen usage but is determined by bullpen usage, meaning that a guy is a closer because he is a closer. But, for the lack of other terminology I am going to use closer and setup guy to mean the two best relievers in the bullpen. I am going to try and use Phillies major and minor leaguers as examples.
Closer: This is purely my definition but a closer needs to have at least either two plus (60) pitches or an average (50) pitch and a plus plus (70) pitch, though if you have a truly dominating 80 pitch (Rivera's cutter) you can survive on that. On top of that need at least average command and plus makeup. I do think there is a "closer mentality" there are just a lot more guys with it then we give credit for. The mentality is one that is a little insane where you think you are invincible and dominating, and then you also to have a short enough memory that you don't get thrown off by a bad outing.
Major League Example: Papelbon - Plus fastball and plus splitter (does have a slider and changeup as well) and plus control to go along with being slightly insane on the mound.
Minor League Example: Kenny Giles - Plus plus to elite fastball that sits 97-99 touching 100 along with an average slider. Still in A-ball working on the command and is a real competitor on the mound
Setup: Missing just one part of that closer profile (maybe just a 60 and 50 pitch or the command isn't there). Still a dominating reliever and often still has "closer makeup".
Major League Example: Phillippe Aumont - Not in that role right now but he has a 70 fastball and 70 curveball (and a mostly shelved 60 splitter), but the command just isn't there to be a closer right now.
Minor League Example: Justin De Fratus - De Fratus has a fastball and slider that are plus to just below with control that is also in the same range.
Middle Reliever: There are some definitely flaws to this pitcher but he can get batter of both sides out pretty well, may show some platoon splits but isn't exposed against opposite handed pitching. Can give you 1-2 innings but shouldn't be counted on in high leverage situations but shouldn't kill you in any game. Utlimately fairly fungible.
Major League Example: Horst - Three pitch mix of fringy to average pitches means no platoon splits. Knows how to use them and can get you multiple innings if you need.
Minor League Example: Too many to list - Every team has a host of guys in the minors who could be middle relievers.
Long Man: This is a dying breed of pitchers. They can also be your #6 starter. This is a guy who can pitch 3-4 innings in a game at a moments notice and won't kill you during them. Often this is a guy who couldn't get through a major league lineup the second time but is just fine the first time through. You never like to see them in a game but you are always glad to have them.
Major League Example - Kyle Kendrick - Before he developed a changeup Kendrick could not make it through a line up multiple times, but he could give you up 6 innings on a moments notice. He was going to give up a run here and there but it wasn't instant death.
Minor League Example - No team is truly brave enough to make this a true role, but there a players out there who would dominate in it.
Righty Specialist: Not a requirement due to the large numbers of right handed pitchers already in a bullpen but this is a guy who devastates right handed hitting but should never face a lefty.
Major League Example: 2009-2010 Brad Lidge - Slider was there when the fastball left and he could still get righties to chase.
Lefty Specialist (LOOGY): There to get the tough lefties out and should never face right handed batting because of gigantic platoon splits.
Major League Example: Antonio Bastardo - He is severely overqualified here because he can get righties but he has LOOGY stuff.
Minor League Example: Jake Diekman - Side arm LHP whose fastball is 94-96 and higher with a good slider. His delivery could really give him problems against righties, but should destroy lefties, still has control issues.
Mop Up Man: Should never pitch in a game closer than 5 runs. Greatest attribute is they can pitch innings. Often the last guy in the pen under the disguise of long man. Normally a veteran of another role or a fringy starter. Hopefully they are cheap.
Major League Example: Chad Durbin
Minor League Example: Tyler Cloyd
We often fall into the trap that relievers are fungible pieces or they are overly valuable. The truth is in between and like many baseball players they are much more effective when used to their strengths. Some teams will overmanage their bullpens with specialists and others will lock players into arbitrary roles they aren't suited for. Teams that manage their bullpens well can exceed expectations and those that don't can lose in agonizing ways. Ultimately the bullpen should be a way to take a flawed baseball player and turn them into a valuable piece in a puzzle