Lester was 3-1 with a 2.41 ERA against the Mariners entering this one. He also had held Seattle to a .229 average, fifth lowest among left-handers who have pitched at least 35 innings against the Mariners.
But the lefty gave up five runs on nine hits in just five innings, reverting back to his June struggles after going 1-0 with a 3.21 ERA in his previous two starts. Prior to those outings, Lester had a string of seven starts where he was 1-4 with a 7.30 ERA.
"I felt like a threw a lot of good pitches tonight, but I threw balls that found the middle of the plate," Lester said. "They did a better job of fouling pitches off to get to those pitches. It goes back to, I feel like I threw the ball better than what the line score says."
The Red Sox (54-37), who own the American League's best record, dropped their third straight game as they fell to 1-3 on the road trip. With injuries mounting in the outfield and bullpen, the recent homestand in which Boston went 8-1 is squarely in the rearview mirror.
"We're a little beat up right now," manager John Farrell said. "We're a little short defensively with position players, with day to day managing that we're doing with some physical ailments. But still, we're in this ballgame in the middle innings and unfortunately it got away from us late."
In a duel between aces, both Lester and Hernandez ran into trouble in the middle innings after breezing through the early frames.
Seattle broke through in the fourth, when Kendrys Morales and Kyle Seager led off with singles to put runners on the corners with none out. Justin Smoak brought Morales home with a double, and the Mariners had two in scoring position with one out. Mike Zunino and Michael Saunders, the bottom two batters in the order, drew walks to bring in another run.
Lester escaped the inning when he got Brad Miller and Nick Franklin on fly balls to end the threat. Still, the lefty needed 39 pitches to get through the inning, ensuring ample work for Boston's depleted bullpen.
"I thought his curveball was the best I've seen as far as being sharp and having some good movement to it," catcher Jarrod Saltalamacchia said. "They took a lot of good pitches, there were some changeups, that normally get swings, they didn't swing at. Some cutters they fouled off, fastballs they fouled off, all you can do is tip your cap."
Boston's potent lineup wasted no time in responding. Jose Iglesias was plunked leading off the fifth and moved to third on Brock Holt's single. Daniel Nava's single scored Iglesias to make it 2-1. Holt scored on a wild pitch by Hernandez during Jonny Gomes' strikeout to tie it.
Raul Ibanez gave Seattle the lead for good in the fifth, hitting a solo home run to right field on an 0-2 pitch from Lester. It was the 41-year old's 22nd home run of the season.
"It was a cutter that got kind of out over the plate," Ibanez said. "I was trying not to do too much. Being 0-2 against a pitcher of that caliber is not the position you want to be in. So I was in survival mode, trying to put it in play and not do too much and I got it in the air to right field."
Lester started the sixth inning with 105 pitches, but gave up singles to Smoak and Zunino before being relieved by Alex Wilson. Saunders brought both runners home with a double, receiving an assist from a throwing error by Nava. Ibanez plated Saunders with a single, putting Boston in a 6-2 hole.
"I think when the relievers came in the game the hitters were comfortable," Saltalamacchia said. "It looked like they were not missing pitches, they were comfortable. We'll talk about it and make some adjustments and figure out how to make them uncomfortable."
Mike Napoli scratched across a run for the Red Sox in the eighth off reliever Oliver Perez, hitting a double to drive in pinch-runner Brandon Snyder. Saltalamacchia brought Napoli around with a double to make it 10-4.
Jacob Thorpe is an associate reporter for MLB.com.
This story was not subject to the approval of Major League Baseball or its clubs.