Rob Refsnyder is the best middle infield prospect in the Yankee system above A-ball, and it isn't close. Refsnyder followed up his breakout 2013 season (.293/.413/.413 between Charleston and Tampa, 148 and 119 wOBA+ respectively) with an arguably stronger start to the 2014 season. He's currently hitting .320/.367/.522 (118 wOBA+) for Trenton.
Those are two very different batting lines. 2013 Rob Refsnyder was a hitter primarily creating value by drawing a lot of walks and making a lot of contact, but hitting for very little power. 2014 Rob Refsnyder is a hitter who is still making a lot of contact, but drawing very few walks and hitting for twice as much power. Look at this peripherals:
Here's the crazy part: Trenton is overall an average ballpark for the Eastern League as far as run scoring for right-handed hitters. However, Trenton has a huge power split for a right-handed hitter. The park has a very strong 109 right-handed park factor for singles, but a below average 95 park factor for doubles and triples, and an absolutely abysmal 79 park factor for home runs. And he's hitting even better at home than on the road.
Essentially, Rob Refsnyder was kidnapped over the winter and replaced with a man who looks like Rob Refsnyder but is a completely different kind of hitter. It's pretty amazing, actually. I can't recall the last time a Yankee prospect transformed his approach so dramatically.
A lot of people like to say that this kind of approach is more sustainable than an approach that is predicated on walks. I haven't seen evidence to prove that, but it does make some logical sense. Refsnyder is controlling the strike zone (15% Ks), but using that ability to drive the ball.
Refsnyder had a great college pedigree for a huge program at the University of Arizona, including leading his team to the 2012 College World Series championship and winning the CWS MVP award. If he keeps on hitting (he batted .354/.381/.619 in May), Refsnyder will be at Triple-A in a heartbeat. Given his experience, pushing him there so quickly shouldn't be difficult.
But if he keeps hitting once he hits Triple-A, the Yankees would be very smart to see if they can catch lightning in a bottle and put Refsnyder on the major league team. Brian Roberts isn't doing much of anything, and Kelly Johnson isn't exactly pushing his way into playing time. The Yankees need a long term option at 2nd base, and Refsnyder is knocking on the door. His obvious (and very optimistic) comp is Matt Carpenter.