Sonny Gray going on the disabled list was said to be a big blow for the Oakland Athletics' rotation given his potential to dominate on the mound. The reality is that he's been a major disappointment and one of their most unreliable starters this season.

It won't be difficult for replacement Zach Neal to better Gray's numbers.

Neal will make his first major league start Wednesday night against Hisashi Iwakuma and the host Seattle Mariners.

Gray had a 2.88 ERA over his first three seasons while emerging as one of the game's best young pitchers, but he struggled out of the gate this year with a 6.19 ERA through nine starts before going on the DL on Sunday with a strained right trapezius muscle.

Enter the 27-year-old Neal, a unproven rookie who won't have to do much to be better than Gray. Neal made his major league debut in relief May 11 at Boston, allowing three runs and four hits in three innings of mop-up duty in Oakland's 13-3 defeat.

The right-hander has pitched well for Triple-A Nashville, though, going 5-1 with a 2.53 ERA in seven starts.

"We like the movement on his fastball and his complement of pitches," manager Bob Melvin told MLB's official website. "We rely on our people in development to have input on that, and Zach Neal's our guy.

"He's pitching well. He's been here and we like what we saw."

Iwakuma (2-4, 4.39 ERA) hasn't been pitching well. He has a 5.19 ERA over his last three starts, showing problems early in two of them and later on in the other.

The right-hander spotted Cincinnati three runs before settling down and lasted six innings to pick up an 8-3 win Friday.

"I was fighting my command early," Iwakuma said. "I was going side to side instead of top to bottom, and that gave me a hard time commanding the ball down in the zone, but I was able to make the adjustment towards the end and it paid off."

Iwakuma was efficient against the A's on May 3, giving up one run in seven innings during an 8-2 victory in his best start of the season. He's 2-0 with a 1.33 ERA in his last three outings against them.

The Mariners (26-19) sit atop the AL West and won 6-5 in dramatic fashion Tuesday when Leonys Martin hit a walkoff two-run homer in the ninth. Seth Smith had three hits, and Robinson Cano hit a two-run shot in the eighth.

It marked another tough loss for the A's (20-27), who have dropped five of their last six after blowing a three-run lead in the final two innings. Coco Crisp had three RBIs before John Axford and Ryan Madson allowed the homers.

Oakland relievers have allowed at least a run in four of the last six and have a 4.06 ERA for the season.