He hasn't been very impressive against the Boston Red Sox, though.The right-hander looks to win a fourth straight start as Seattle seeks its first ever three-game sweep at Fenway Park on Sunday.Iwakuma (12-6, 2.57 ERA) has posted a 0.61 ERA in four outings this month and has 120 strikeouts against only 12 walks on the season. He struck out a season-best 11 with no walks over eight scoreless innings of Tuesday's 5-2 win over Philadelphia."He was outstanding," manager Lloyd McClendon said. "He commanded all of his pitches. He was in and out, up and down. Did a tremendous job for us."Iwakuma hasn't been nearly as good against Boston, going 0-1 with an 8.53 ERA in three starts, including allowing a season worst-tying five runs in four innings of a 5-4 loss June 25 in his shortest outing of the season.David Ortiz is 4 for 7 with two home runs and Yoenis Cespedes is 8 for 22 with three homers and four doubles off Iwakuma.Ortiz had reached base four times in four straight games prior to Sunday, when he walked and was hit by a pitch before leaving a 7-3 loss early with a bruised left elbow.Dustin Ackley hit a three-run homer as Seattle (70-58) overcame a three-run deficit with a seven-run fourth inning. The Mariners have a slight edge over Detroit for the second wild-card slot."I don't care how we get (runs) as long as we get them," McClendon said. "That was nice, to bounce back like that."Seattle entered this series having lost nine straight at Fenway Park, and it has never earned a sweep of at least three games there since entering the league in 1977.It will look to end that streak as it faces Allen Webster, who hopes to have better control in this contest.Webster (3-1, 4.73) had a solid outing Tuesday, giving up three runs in six innings of Boston's 4-3 loss to the Los Angeles Angels. He's pitched at least six innings in his last three starts."It's good to see him continue to back up outings in a positive way and build some momentum," manager John Farrell said. "I'm sure (he has) some confidence in his own right."The right-hander walked two to give him 18 compared to 13 strikeouts in five starts."I've just got to keep telling myself to trust my stuff," Webster said. "Let them put the ball in play."The Red Sox (56-73) have had some trouble doing that themselves lately, getting outscored 38-15 during a seven-game losing streak, with each defeat coming at home.Boston could get a lift at some point from Cuban defector Rusney Castillo, whose seven-year, $72.5 million contract officially was announced Sunday. Farrell said he's hoping Castillo can get some at-bats in the minors before playing for the Red Sox."Above-average speed, can play center field or right field," Farrell said. "What kind of power? What kind of average? Obviously, our scouts liked him enough. If the reports are true, it's a significant investment and it's an exciting, athletic player, from all accounts."