Hitters usually do not change. Hitters who are selective at the plate generally stay selective from year one to year two, and aggressive hitters usually remain aggressive from year to year as well. That is why Alex Rodriguez is such an interesting case, because not only do older players exhibit issues with plate discipline, but players with a year off certainly have problems with pitch selection and timing. He came into this season with a 12.7% walk rate and 23.8% strikeout rate in 2013, and he now sits (as of Monday afternoon) at a 14.2% walk rate and 21.7% strikeout rate. And that's not the only place where his discipline has remained similar, in terms of his contact percentages:
That is remarkably similar when comparing 2013 to 2015, and this is the type of data that stabilizes incredibly quickly, as soon as 50 plate appearances, in fact. The real question regarding this relative stability is: is this real? I think it would be interesting to see if he is actually laying off of bad pitches, and here is a comparison from 2013 to 2015 in Swing%:
The difference is subtle, but there certainly is one. He is focusing on swinging on pitches that are up in the zone, and his bat speed is fast enough that he able to catch up to hanging breaking balls and fastballs alike up in the zone, especially up-and-in. The Swing% has increased in the lower part of the zone, and that could be where his issues lie. He has had problems with off-speed pitches low, and that is likely to continue.
Is there any qualitative evidence that this should continue? Possibly. Before the season, A-Rod had worked with Barry Bonds, the all-time great who also has the record for the most walks in baseball history. It was said in the winter that:
"According to the San Francisco Chronicle report, Bonds has tutored numerous players at the Future Prospects facility in San Rafael, Calif., the past couple of offseasons. And A-Rod, who recently posted the photo below on his Instagram account, was among his pupils this month.
I don't really know what this means. Players are constantly making adjustments, and no one really knows whether these adjustments are just evidenced because of a random stroke of luck, or because it actually changed the way they play in a positive way. We'll always quote the "great hitting coach" when a player is hitting 139 wRC+, but rarely when he's hitting below league average.
But if A-Rod's bat speed is real--which I think it is--then I don't see why his plate discipline numbers cannot stay relatively similar over the course of the year. Both ZiPS and Steamer think the walk rate will drop to about 10%, and the strikeout rate will settle around 24%. I would buy that, and I could also see the walk rate around 12%. It's very difficult to tell whether his improvements are real, but I think it would be fair to take the mean of the current and projection.
A-Rod has already proven that he is better than we thought, at least by a few ticks. Projection systems think he is actually a true talent hitter of slightly above league average, and I generally agree. He could even be a 110 wRC+ hitter, at least if he stays healthy. And as far as plate discipline goes, it works the same. It is certainly possible that the lower half of the zone could partially erode his gains, but it is likely that he is closer to his 2013 self than we thought. And for the Yankees, that is a huge win.