Hernandez (8-10) walked Shin-Soo Choo to get to Hafner with the bases loaded in the seventh inning. Gutsy, but definitely not smart.
"They walk Choo to get to you and you take it personal," Hafner said. "When that happens, you want to go up there and do some damage."
Hafner connected off Seattle's ace after a two-out error by second baseman Chone Figgins opened the door for Cleveland's seven-run seventh.
"Two outs, we make a mistake and they made us pay," said Mariners interim manager Daren Brown, after dropping to 4-2 since replacing the fired Don Wakamatsu on Monday.
"A not-so-good homestand, but just when you think you've seen it all" Indians manager Manny Acta said. "I don't think I've seen a guy go six innings, give up one hit, and throw more balls than strikes."
Masterson walked six and only 49 of his 100 pitches were strikes.
"I had a lot of movement on the ball," Masterson said, "but it was never like, 'Oh, man, I can't find the zone.' It was a lot of close pitches. I just tried to hang in until the offense did something."
Hernandez was sailing, too -- until Luis Valbuena reached when Figgins bobbled his grounder, then threw high to first baseman Casey Kotchman. Replays showed Kotchman kept his foot on the bag, but umpire Todd Tichenor ruled otherwise.
"I should have made the play," Figgins said. "It took a hop on me, but I should have made the play."
"Great hustle by Valbuena, a tough at-bat by Marson and Brantley battled back from an 0-2 count to get a big hit," Acta said.
Asdrubal Cabrera singled home Marson and with runners on second and third, and Hernandez intentionally walked Choo.
Hafner, who had been sidelined with a tired right shoulder, drove a 2-1 pitch 420 feet to center for his second slam of the season and 11th of his career. Hafner was 0 for 3 with two strikeouts until then in his first action since July 29.
"I was thinking [right then] I should have gone on the rehab," Hafner joked.
Hernandez was coming off a 2-0 win over Oakland on Tuesday in which he struck out a career-high 13 and yielded five hits over eight innings. Seattle has totaled five runs in his last six starts in which the right-hander is 1-4. The Mariners have 10 total runs in his 10 losses.
The 24-year-old dejectedly sat at his locker and was consoled by teammates before talking about Hafner's homer.
"It was a sinker," Hernandez said. "It was a little bit up. He put it over the fence. I knew it was gone. It's 2-0, we have a chance to come back and win the game, but 6-0."
Hernandez turned silent before adding that he didn't blame Figgins.
"Errors are part of baseball," Hernnadez said. "Anyone who doesn't make an error is watching from the stands."
Masterson matched King Felix in putting up zeroes -- with defensive help from first baseman Matt LaPorta.
Through four innings, the only baserunners came on three walks by each right-hander. Kotchman got the first hit, a double to open the fifth.
LaPorta then bailed out his pitcher with two fine plays. He made a diving catch of a line drive by Ichiro Suzuki that was headed for the right-field corner. He then fielded a bunt by Figgins and tagged the speedy runner as he sped past.
Marson broke an 0-for-14 slump with a two-out double in the bottom half for Cleveland's first hit.
Franklin Gutierrez had an RBI single in the Seattle eighth, while Marson singled and scored on Brantley's third homer to make it 9-1 in the bottom half.
Choo got his 10th assist, tying Toronto's Jose Bautista for the major league lead among outfielders. Choo threw to first base after catching a fly ball to double off Russell Branyan. Cleveland outfielders have 25 assists overall. ... Cleveland optioned rookie OF Jordan Brown to Triple-A Columbus to activate Hafner. Brown, called up July 31, hit .226 with one RBI in nine games. ... LaPorta is in an 0 for 20 slump. ... Seattle has lost 13 of its last 18 road games.