The clearest window into the future of Yankee first-round draft pick James Kaprielian may not be how he pitched in three years at UCLA, but rather during one summer on Cape Cod.

The Cape Cod Baseball League was a proving ground for 142 current major-league pitchers, and few were as dominant there against the best amateurs in the country as Kaprielian, when he pitched for Yarmouth-Dennis in 2013. The 16th overall pick in Monday’s MLB draft made the all-star team that year on the strength of a 1.80 ERA and a rate of 12.6 strikeouts...