Not a natural at fielding ground balls. He's out of position too often, ends up in an awkward position. Like on the soft ground ball this game with the messed up flip to Polanco. It was probably too soft to turn 2 anyway, but he has to charge that and field it with his throwing hand right next to his glove, so he can make the shuffle to second in a flash. His throwing hand was a mile away as he stabbed at it, so it would have taken way too long to make the exchange, even if he'd done so cleanly.
Just a lot of little things like that. Instincts, coordination, natural ability to put yourself in a position to field ground balls, time and again, and turn them into outs. It's a hard thing to do.
3rd base is so much easier. Reaction, which I think he has.
If course, I'm making this judgement based on a handful of games. I could end up wrong.
But as I see it, he's not going to stick.