The Mariners announced Iwakuma's surgery on Friday. The shoulder debridement was done Wednesday in Dallas by Dr. Keith Meister. Iwakuma could resume throwing in late February or early March.
Iwakuma had a contract option for the 2018 season but needed to pitch a specific amount of innings for the contract to vest. His injury ended that chance, and he is expected to be a free agent after the season. The team has a $10 million option on him for next season that it can buy out for $1 million.
Iwakuma, 36, had a frustrating, injury-filled 2017 season. He made just six starts and was 0-2 with a 4.35 ERA. He landed on the disabled list May 10 with shoulder troubles and never made it back to the roster.
Iwakuma has spent all six of his major league seasons with the Mariners.
The Associated Press contributed to this report.