The 32-year-old appeared in 28 games for the Yomiuri Giants in 2019. He finished 16-4 with a 2.78 ERA and averaged 9.6 strikeouts per nine innings.
The right-hander began his career in 2006 as a starter before moving to the bullpen full-time in 2008. He assumed a starting role again in 2014. Over 14 seasons in Japan, he's 65-58 with 112 saves and a 3.33 ERA.
Yamaguchi announced in November in intended to make the jump to MLB.
"I will take a shot at my dream of playing in the majors," he said, per the Japan Times. "I'd like to express my appreciation to Yomiuri Giants officials, manager Mr. [Tatsunori] Hara, my coaches, teammates and fans for the past three years. I will work even harder as I pursue a new challenge."
MLB.com's Jon Paul Morosi noted Yamaguchi was one of several Japanese players who'd be available to sign this offseason:
Because of his age, Yamaguchi wasn't going to set off a frenzied bidding war similar to that of Masahiro Tanaka and Shohei Ohtani. Tanaka was 25 when he was posted in December 2013; Ohtani was 23 when he left Japan in November 2017.
But interested suitors had a large body of work from which they could project Yamaguchi's value.
According to Japanese baseball writer Jim Allen, Yamaguchi threw his fastball 43.7 percent of the time last year and averaged a little over 90.1 mph. Allen also provided some takeaways from his 2019 performance:
"On a pitch-per-pitch basis, his splitter, again according to Delta Graphs, was the most effective in NPB among pitchers with 80-plus innings. Although his fastball is not an out pitch for him, Yamaguchi's success with his splitter this season and to a lesser extent his slider is probably related to an uptick in his four-seam fastball velocity."
Allen projected Yamaguchi could operate as a back-of-the-rotation starter or a reliever in MLB.
Not every Japanese pitcher can immediately make the transition to the majors.
Yamaguchi's inexperience in the States obviously brings a level of risk, but the Blue Jays aren't bringing him in to be the ace of the staff. As long as he can perform like a mid-rotation starter, he'll justify their investment.