Iwakuma, 31, pitched against Darvish six times in Pacific League games in Japan and the two were teammates on Japan's 2008 Olympic team and the 2009 World Baseball Classic championship squad.
This will be the first time they've competed against each other in the U.S., with Darvish holding a 3-2 edge in wins in head-to-head games in their homeland.
"We are friends, and sometimes we get dinner together and sometimes we talk on the phone," Iwakuma said through interpreter Daisuke Sekiba. "I'm looking forward to pitching against him. Of course, it's fun waiting to face each other at the Major League level. In Japan, we played against each other a couple games. So we expect that, it's just now we're here."
Darvish, 26, received far more attention after signing a six-year, $60 million deal with the Rangers and has put up a 14-9 record with a 4.14 ERA for the AL West-leading Rangers. But Iwakuma has quietly made a mark with the Mariners after agreeing to a one-year, $1.5 million deal.
Since moving into the rotation in midseason, he's gone 5-3 with a 2.80 ERA in 12 outings, including a 4-0 loss to the Rangers in his second start in July. That game was in Seattle and now he faces the potent Texas lineup in hitter-friendly Arlington.
For Iwakuma, that's a more significant challenge than facing his friend from Japan.
"It's different, because I'm going to pitch against the Rangers," he said. "That's the biggest difference."