"I was throwing pretty good. I was commanding the strike zone with men on base. But that one pitch, if I could take it back, I'd take it back."
Hernandez is now 3-2 on the season with a 2.29 ERA. He'd gone seven innings or more while allowing two runs or fewer in his five previous outings.
Montero, making his first appearance in New York since being traded for Michael Pineda in January, drove an opposite-field blast off Hiroki Kuroda leading off the sixth for a brief 2-1 lead.
"I was thinking the whole time, right field, as soon as I stepped the first time to the plate tonight," said Montero. "Right field is right there. So I was trying to hit the ball over there every time. They were making a lot of good pitches, so it was a little tough, but I finally got it over there and hit that one pretty good."
The 22-year-old, regarded as the Yankees' premier offensive prospect for the last several years, now has five home runs and 17 RBIs while batting .267 in his first 30 games with Seattle.
But Hernandez couldn't hold that advantage, as his former teammate, Ibanez, ripped a three-run blast in the bottom of the sixth, and the Yankees (18-14) spoiled Seattle's start to a 10-game, four-city trip. The setback was the seventh straight road defeat for Seattle, which fell to 15-19.
The loss was Hernandez's first at the new Yankee Stadium, which opened in 2009. He'd allowed just one earned run in three prior starts, but is now 3-1 with a 1.46 ERA and 34 strikeouts in 30 2/3 innings at the facility.
Overall, Hernandez is 6-5 with a 3.45 ERA in 12 starts against the Yankees.
An RBI single by Robinson Cano in the first inning was the first opening-frame run given up by Hernandez since July 22, 2011, against the Red Sox, a club-record span of 18 starts.
The Yankees strung together three straight singles leading off the fourth, but left fielder Mike Carp threw out Alex Rodriguez trying to score from second on Mark Teixeira's base hit. Nick Swisher advanced both runners on a nubber down the first-base line before Hernandez struck out Ibanez to escape that predicament.
But he couldn't get out of similar trouble in the sixth when Ibanez stroked the first pitch he saw over the right-field fence for his sixth home run of the season, the three RBIs raising his total to 19.
"I think when you're facing an elite pitcher like him, sometimes the first pitch is the best one he's going to throw you," said Ibanez. "Then he gets nasty. That's what he was doing. I think it was just kind of the mindset that, 'He's a great pitcher, so whatever you see that's good to hit, jump on it.'"
Andruw Jones added two runs for New York with a pinch-hit home run off reliever Steve Delabar in the eighth.
The Mariners countered with a pair of home runs of their own, but both were solo shots. Dustin Ackley led off the game with an opposite-field poke, his first home run since Opening Day in Tokyo, before Montero added his own shot in the sixth.
Seattle had two on with no outs in the second against Kuroda before Justin Smoak, Carp and Michael Saunders went down in order. They had another golden opportunity with the bases loaded and one out in the fifth, but Brendan Ryan struck out and Ichiro Suzuki grounded to third.
"In a tight ballgame like that, you squeeze even one run across both those innings, and it's a different feel, a different ballgame," said manager Eric Wedge. "We didn't get it done."
Seattle finished 0-for-5 with runners in scoring position and 1-for-12 with men on base. Smoak did break out of a 3-for-30 slump by going 3-for-4 with three singles to raise his season average to .193.
"I felt good at the plate tonight. I wish I could have that first at-bat back with runners in scoring position and no outs," said Smoak. "You get a fastball there over the plate, you've got to do something with it. But I'm starting to feel a little better."