[1] The Yokohama BayStars selected Yamaguchi in the first round in the 2005 Nippon Professional Baseball draft.

In 2016, Yamaguchi earned all-star honors for the third time, notching an 11-5 record and 2.86 ERA on the year.

In 2019, he posted a 15–4 record (.789 winning percentage) that led Japan's Central League, along with a 2.91 ERA and 188 strikeouts over 170 innings.

[6] After the 2019 WBSC Premier12, on November 18, 2019, Giants held a press conference announcing the team has allowed Yamaguchi to enter the posting system to play in Major League Baseball (MLB).

[5] On December 28, 2019, Yamaguchi signed a two-year, $6.35 million contract with the Toronto Blue Jays.

Yamaguchi was issued uniform # 1, being only the fifth Toronto pitcher (not initially listed as a position player) to ever pitch in a game wearing a single digit uniform number after Josh Towers, Kyle Drabek, Marcus Stroman, and Clayton Richard.

[18] He became the first Japanese player who made his MLB debut as a Blue Jays member.

[19] With the 2020 Toronto Blue Jays, Yamaguchi appeared in 17 games, compiling a 2–4 record with 8.06 ERA and 26 strikeouts in 252⁄3 innings pitched.

[22] On February 20, 2021, Yamaguchi agreed to a contract with the San Francisco Giants, reportedly a split major league/minor league deal.

[23] On June 2, Yamaguchi announced via Instagram that he would be leaving the Giants organization to return to Japan.

On June 10, 2021, Yamaguchi agreed to a contract to return to the Yomiuri Giants of Nippon Professional Baseball.