KORYO, Japan—On the mound of a floodlit baseball diamond, Shota Tatsuta began another day of pitching practice.

He paused every now and then to signal with his hand the next pitch to the catcher—a slider, then a fastball. The other players, ready to go home, were grooming the field with rakes and cracking jokes as the day came to a close.

Two months had passed since Shota’s high-school baseball career ended in defeat, when his Yamato-Koryo High School got knocked out of a crucial summer regional tournament and failed to make it to Japan’s famous Koshien national baseball championship.

Shota, an 18-year-old pitching phenomenon profiled by The Wall Street Journal in July, drew national attention as much for his contrarian ways as for his rocket arm. Unlike other young Japanese pitchers, who court injury by throwing as many innings as possible to win and prove their endurance, Shota pledged to protect his arm the way many American pitchers try to do, only throwing when absolutely necessary.

But when faced with the pressure of winning a big tournament to make it to Japan’s national championship, he wound up throwing 463 pitches over eight days—a number far above limits recommended by American coaches.