Nagashima entered Rikkyo University, following his father, and attracted much attention from professional scouts, being the son of Shigeo Nagashima. He was drafted in the first round of the 1988 draft by the Yakult Swallows, and his first professional hit was a home run against the Yomiuri Giants, and the pitcher was Bill Gullickson. He was touted as the next big star in Japanese baseball, after his father, but failed to live up to expectations. Nagashima did not hit or field well, and his playing time decreased as the years went on. In 1992, he spent a year with the Los Angeles Dodgers single-A team in the United States, but made little improvement.