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The knock on Seager is his range at 2b ... steady hands, few mistakes, but not super quick compared to some MLB gloves there.
If you've got the arm, then your mediocre 2b play becomes --- > plus 3b play. Teams been doing that for a hundred years.
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Ordinarily you want to keep an eye on PWR in the corners, but (1) that doesn't apply if you are REALLY talking about a top-20 OBP, and (2) Ackley's power at 2b somewhat balances an OBP-first player at 3b.
At the moment, I'm intrigued by Seager at 3b, and I'm not sure that wouldn't mean a trial this year. They've got to get data on Seager before they have to decide on Nick Franklin, Carlos Triunfel, etc.