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Ever heard a pitcher say "I couldn't find the feel for my curve/change/slider today"?
Pineda had his hands full as it is, fine-tuning his 2-pitch sequences and keeping his mechanics consistent. It's similar to stripping down an NFL offense so that a new QB can execute an offense.
You have two 80 pitches, and you can add another 70 on the fly, without confusing the kid, fine. But why would the 1997 Kerry Wood have wanted to mix a 40 changeup in with his 80 fastball and 80 slider?
...........
Paxton, with his incredible mechanics, probably could throw the changeup without taking away from his fastball and curve.
But they mess around and try to turn Taijuan Walker into John Lackey, they're going to be sorry.