Recent changes in U.S. immigration law and growing competition in baseball for raw talent have allowed the minor-league farm system to flourish with imported players. It has been a home run for globalization, but bad news for U.S.-born players, who suddenly have much more competition. Across the minor and major leagues, the total number of foreign-born players is growing fast, to almost 3,500 of the 8,532 players under contract this summer, from 2,964 three years ago.

Boise Hawks' hitting instructor, Ricardo Medina, a native of Panama who translates at team meetings in what has become almost a bilingual program, notes that Mr. Lee and his Korean teammates are getting something else from their summer in Idaho. "I think they may be learning more Spanish than English," he jokes.