Shōta Imanaga
The next-best Japanese pitcher available on the market was Shōta Imanaga, who has pitched eight impressive seasons in Japan and was the starting pitcher for the World Baseball Classic championship game against Team USA.
With the time running out on his posting window, Imanaga signed a unique four-year, $53 million contract with the Chicago Cubs, securing his jump to the majors at age 30.
The Red Sox were very much in the running to secure Imanaga's services. According to Alex Speier of the Boston Globe, the Red Sox had offered Imanaga a two-year contract with two additional vesting years, not far off from what he ended up signing for with the Cubs.
All offseason, the Red Sox have been unwilling to offer long-term contracts to players to the frustration of the fanbase. A four-year deal for Imanaga isn't a huge commitment, and at the money he signed for, the Red Sox shouldn't be let off the hook for not matching that offer to try and land the southpaw, especially with all the candidates for rotation spots right now being right-handed.