Iwakuma pitched eight effective innings to help the Mariners stop a five-game losing streak with a 5-1 victory against the Texas Rangers.
"I needed to pitch today. I felt responsible, especially after losing five in a row," Iwakuma said through an interpreter.
It was quite a turnaround for Iwakuma after he felt stiffness in his neck as he prepared for his ninth start of the season.
"To be honest, yes, as I was playing catch before my bullpen I did feel that way," Iwakuma said of having doubts he would be able to pitch.
Iwakuma, who didn't make his first start of the season until May 3 due to a finger injury on his throwing hand, improved to 2-1 with a 1.61 ERA in three starts this month.
"My body's starting to get used to season mode and I'm feeling a lot better and stronger now, so I think I'm good to go from here on," said Iwakuma, who was hopeful his neck wouldn't be a problem moving forward.
The Mariners left a runner in scoring position in each of the first four innings before Seager drove in Endy Chavez and Robinson Cano with a two-out double in the fifth, lifting Seattle to a 2-1 lead.
"It's been a little bit of a grind as of late, but before that we were playing really good baseball," Seager said. "That's just kind of the way baseball goes. You go from scoring runs all the time to kind of hit a little rough patch, but that's just kind of one of the things you grind through it."
Seager added an RBI double in Seattle's three-run eighth as the Mariners avoided being passed in the standings by the Rangers.
Seager has played well against the Rangers in his career, hitting .338 with 34 extra-base hits and 38 RBI. Seager is hitting .396 (19 for 48) this season against Texas with three home runs and 12 RBI.
Rangers starter Nick Martinez (1-4) allowed nine hits and walked one, but held the Mariners to two runs in six innings. The right-hander is 0-3 with a 9.45 ERA in his last three starts.
"He found his secondary stuff today," Rangers manager Ron Washington said. "Threw a lot of changeups, threw some breaking balls. And with Nick, you need all of his pitches to maneuver through the batting order and they did. He made the mistake to Seager but certainly a big improvement."
Mariners 1B Logan Morrison departed after five innings due to a cut above his left eye that needed five stitches. Morrison was hurt when he swung his bat in frustration at the dugout wall and had a piece fly back and hit him. Catcher John Buck replaced him in his first major league appearance at the position. ... Mariners 1B Justin Smoak (strained quad), OF Corey Hart (strained hamstring) and OF Michael Saunders (shoulder inflammation) are close to being sent out on rehab assignments as they work their way back from stints on the disabled list, manager Lloyd McClendon said. The Seattle skipper said all three could begin a rehab assignment by week's end. ... Texas OF Leonys Martin occupied the top spot in the lineup for the second time in as many days on Sunday as the Rangers try to find a consistent leadoff hitter. Washington said that doesn't necessarily mean he has secured that role, though.