Does pitch differentiation matter, or just the amount of different pitches?

[Zooerp]

For example, if this draftee's slider develops, can he be a successful starter (as three pitch staters go anyway)? Typically a fastball, cutter, slider guy would be a reliever, but does the fact that a fastball and cutter are similar pitches matter in OOTP? I usually try to find guys with a fastball variation, curve variation (slider, curve, slurve), and a changeup variation (change, split, forkball) to start.

[CBeisbol]

There's a link in my signature to user studies. One study is about pitch types. There may be some information there that would be useful to you

[Syd Thrift]

That guy also has a “show me” change up so I think that he’s got a good chance of becoming a starter. That said, I think what might hold him back from being a solid starter if the change doesn’t develop isn’t so much that he only has 2 fastball-ish pitches and a breaking ball but that none of the pitches are really that great. In order for a starter to be effective throwing only three pitches, all three pitches need to be really good. I don’t think one 65 and a couple of 55s is going to cut it. With the change and above average control I can see how they’d get to the 50 potential the game sees (which is probably still only around 3rd starter level at best) but all 4 pitches have to develop.

To your larger question, no, I don’t think a pitcher needs a fastball, change, and breaking ball in order to become a good starter so much as they need 3 excellent pitches or 4 or more good ones, period. It’s very rare that you’ll see a guy with 4+ pitches who doesn’t have one in each category though; off-hand the only type I ever see like that are guys without a fastball, namely knuckleballers, and those guys practically adhere to a different standard (they tend to go knuckleball/knuckle curve/change, and while their stuff ratings tend to be low-ish, the knuckleball has an effect on BABIP).