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If Ackley and Carp give you two more consistent XBH + BB guys, then you probably can afford to have a random-moon-shot guy in the lineup, too. And we have two candidates for the job -- or they can share it.
Yeah, Carp has less chance to be special than Rendon, but may end up being as productive. I wouldn't trade Rendon for Carp in a mythical world in which it were possible, but Carp is making it more interesting than expected.
And, actually there are seven guys on the roster 25-and-under: Felix, Pineda, Smoak, Ackley, Carp, Peguero and Halman. (Felix was 25 in April, Carp is 25 in a couple of weeks.) And Saunders and Cortes have been up, if you still want to count them.