With the worsening of Jose Reyes' injury, Alex Cora figures to inherit the starting shortstop job for a considerable portion of the season going forward. So far, Cora has filled in admirably, posting a .394 wOBA. His .370 BABIP is high, and his average will probably drop off a bit soon. The good news, however, is that Cora's plate discipline to this point has been fantastic, earning him the over-referenced "professional hitter" label. With 10 walks in just 66 plate appearances, it might be easy to dismiss the improvement entirely as a sample size fluke and default to his not-so-good 6.9 % walk rate and .293 wOBA. Given that he posted a solid 1.23 in the tougher AL East last year, however, Cora may well be one of many players who improve their plate discipline as they age. Cora won't maintain a .394 wOBA, but he could well beat his ZiPS rest-of-season .324 projection. For our purposes, .330 wOBA (league average) seems like a good estimate.
Assuming Jose Reyes' .370 wOBA rest-of-season projection, the difference between them can be calculated simply as: .37-.33=. 04 *PA=R. By those estimates, for every 100 PA Reyes plays over Cora, the Mets lose 4 runs or (4/10) of a win. We'll have a better idea of exactly how long Jose will be out soon enough, and you can make your own PA estimations.
The wild card in this swap is Cora's fielding. As Eric pointed out earlier, Cora's 7.8 UZR/150 is much better than Reyes' 1.9. Still, Jose's UZR yearly numbers have fluctuated from 7.5 to -3.2 in individual seasons, so it's hard to peg him just yet. He was -1.2 so far this year, in a virtually meaningless sample size. Cora, however, has seen his numbers decline a little lately, as UZR measured his shortstop defense as negative for the first time last year. Still, a year's worth of fielding data is equivalent to roughly a third of a year's worth of hitting data, in terms of statistical significance, so his career numbers may be the most accurate reflection of his ability. Alex Cora will never fully replace Jose Reyes, and just because we temper our expectations for him, doesn't prove some fictional stat/scouting divide. Regardless, he's a good backup, and just how much of Jose's value he recoups may depend largely on how sharp he is in the field.