Iwakuma reported for his physical Wednesday at Mariners spring training headquarters and had a splint over the finger. He's been diagnosed with a strained tendon and will be re-examined by a hand specialist in Arizona in three weeks.
The injury isn't expected to require surgery, at least for now, but he's likely out for all of spring training.
Iwakuma said his finger got caught as he tried to retrieve a ball in some netting while working out in Southern California on Jan. 20.
"I knew it was injured. I didn't think it was a major thing," Iwakuma said through team translator Antony Suzuki. "I rested my finger for about a week and started playing catch again, long toss. I thought it was good but then the pain didn't go away."
Along with ace Felix Hernandez, Iwakuma has given Seattle a formidable duo at the top of its starting rotation. He went 14-6 with a 2.66 ERA and 185 strikeouts in 2013.
"Luckily, this does not appear to be a serious injury," Mariners general manager Jack Zduriencik said in a statement. "It is a setback for 'Kuma,' but we are confident that he will quickly overcome the missed time and be able to rejoin our rotation early in the regular season."
Iwakuma said he wants to be ready for the season but isn't sure it will happen.
"To be honest, very disappointed with what happened," Iwakuma said. "We have to go from here on. I look forward to helping the team in any way and I look forward to coming back as soon as possible, too."