The 36-year-old Iwakuma made only six starts last season, going 0-2 with a 4.35 ERA. The problems with his pitching shoulder sent him to the disabled list in May and caused setbacks during his recovery.
He had an operation in late September.
"I've got my fingers crossed," Mariners manager Scott Servais said Tuesday at baseball's winter meetings. "He's a huge asset and we'll hope for the best."
Servais added that Iwakuma's return could happen "probably" in mid-May to early June.
"We really missed him last year, and what he's able to do, and just his consistency of going out there and giving you six competitive innings," Servais said.
Iwakuma has spent six seasons with the Mariners. He is 63-39 with a 3.42 ERA in the majors.