In each of the past two seasons, Chicago White Sox rookie infielder Marcus Semien has recorded among the most impressive walk and strikeout rates in the high minors. Insofar as it’s generally the case that walk and strikeout figures from the high minors translate somewhat predictably to the majors, a reasonable person — and also the present author — might reasonably have expected Semien to approximate those same figures at baseball’s highest level. That he absolutely hasn’t presents a bit of a mystery.

Last year, for example, he recorded walk and strikeout rates of 15.1% and 15.8%, respectively, in 714 plate appearances between Double- and Triple-A. An encouraging performance, that. Following a late-season promotion to the parent club, however, Semien produced figures of just 1.4% and 31.0%, respectively, in 71 plate appearances. Nor has he been immune from a similarly robust deterioration in plate-discipline/contact skills this season. Regard, firstly, Semien’s walk and strikeout figures in 366 plate appearances with Triple-A Charlotte: 14.5% and 16.1%, respectively — which is to say, almost identical to his minor-league numbers from 2013. Meanwhile, his walk and strikeout rates from 205 PAs with the White Sox? Just 7.8% and 30.2%, respectively.

Here’s roughly all that same information, except in table form and featuring Semien’s minor- and major-league data from 2013 and -14 combined:

One finds that, over the last two seasons, Semien has produced a walk and strikeout differential of just -1.0 percentage point in the minor leagues, but of nearly -30 points in the majors. To get a sense of how a player with Semien’s minor-league track record might perform in the majors, consider his Steamer and ZiPS projections. Both systems call for Semien to walk in about 10% of plate appearances and strike out in about 21-22% of them — roughly league-average figures, both of them. Semien’s actual plate-discipline numbers, though, are considerably below average.

To get a sense of where Semien might be having the most difficulty, I’ve embedded the following heatmap featuring his contact rate by zone during 2013 and -14. For every zone within which Semien has recorded a sample of at least 10 swings, I’ve included the difference between Semien’s own contact rate and the league-average one.

The space outlined in a cloying shade of green represents that area within which Semien has had some success. On pitches roughly in the middle of the zone, vertically speaking, Semien has produced roughly average contact rates relative to his major-league peers — and has even outperformed them on pitches low and outside. High strikes have posed a considerable problem, however: here Semien has recorded contact rates about 12 percentage points below league average. Lower pitches on the inside part of the plate also appear to have caused Semien some troubles.

Further consideration by pitch type might serve to understand even more about Semien’s surprisingly robust deterioration of plate discipline at the major-league level. That the author is writing even these words after 5pm on a Friday, however, suggests that such an inquiry will only appear in a sequel.