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Zuumball's energy in general, and he obviously was positive about our boy.
Not to sound off-tone but a cut fastball is not rocket science; if you have a gloveside fastball, it's going to set up your armside fastballs. Question in my mind is how many cutters a game Marc-O's elbow can take.
First indication on that will be the F/X against the Giants. He only needs about 12 per game if judiciously used. Cutters can be supplemented a bit with a tight-spin slider like Marc-O's.