Iwakuma reported for his physical Wednesday at the Mariners' spring training headquarters and has 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.
Iwakuma said he caught his finger as he tried to retrieve a ball from some netting while working out in Southern California on Jan. 20.
Iwakuma went 14-6 with a 2.66 ERA and 185 strikeouts in 2013.