The Mariners put Paxton on the 15-day disabled list on Friday with a strained middle finger tendon on his throwing hand. He felt it after 2-0 pitch to Brandon Moss in the fifth inning Thursday and was immediately removed.
Iwakuma went on the disabled list on April 24 with a strained right lat dorsi muscle and has missed 32 games.
''It's a tough loss, obviously. When you think we've lost our No. 2 and No. 3 starters, that's tough,'' Mariners manager Lloyd McClendon said. ''But in this game no one feels sorry for you. ... We just have to figure out the next man.''
The Mariners called up right-hander reliever Dominic Leone. The club will need to make a move by Tuesday when Paxton's next start is scheduled.