Iwakuma debuted in 2012 and went 9-5 with a 3.16 ERA. In the year after, he would have the best season of his career in the United States, as he went 14-6 with an All-Star Game appearance and a 2.66 ERA with 185 Strikeouts. Those numbers were his best in the Majors, and he would also lead the American League in Pitchers with bWAR with 7.2. He would finish third in Cy Young voting that year.
While the Japanese hurler would never hit those numbers again, but he would have two 15 Win years (2014 & 2016), both of which seeing him exceed 140 Ks. Iwakuma would be injured through much of 2017 and 2018, while also seeing duty in the minors. He would return to Japan to play baseball in 2019.With the Mariners, Iwakuma went 63-39 with 714 Strikeouts.