BOSTON -- A confusing sequence in the bottom of the sixth inning proved costly for the Mariners, who fell to the Red Sox, 12-6, in the first of a four-game series at Fenway Park on Thursday night.

• Box score

With the game tied at 4-4, the Mariners sent starter George Kirby out to warm up in the sixth, only for manager Scott Servais to relieve him before he faced a batter.

As Kirby went to the dugout and Sergio Romo made his way to the mound from the bullpen, the umpiring crew had an animated meeting in the outfield grass before quickly determining that the Mariners were in violation of Rule 5.10 (h) (i).

Rule 5.10 (h) (i) - If a pitcher who is already in the game crosses the foul line on his way to take his place on the pitcher’s plate to start an inning, he shall pitch to the first batter until such batter is put out or reaches first base, unless the batter is substituted for, or the pitcher sustains an injury or illness which, in the judgment of the umpire-in-chief, incapacitates him from pitching. If the pitcher ends the previous inning on base or at bat and does not return to the dugout after the inning is completed, the pitcher is not required to pitch to the first batter of the inning until he makes contact with the pitcher’s plate to begin his warm-up pitches.

Forced to come back to the mound after reaching the dugout, Kirby allowed a leadoff double to Alex Verdugo on his only pitch in the inning before Romo came on. Trevor Story, who had already homered twice, then laced a single to center field that was scooped up by Julio Rodríguez, who threw wildly to third, allowing the go-ahead run to score.

“The momentum of the game really changed there, and they added on after that,” Servais said.

Following the game, Servais took full blame for the incident, explaining that he was trying to give Romo extra time to get ready. He added that he had conferred with home-plate umpire Scott Barry prior to sending Kirby back out.

“Before any of that happened, I went out to talk to the home-plate umpire to make sure I was good to go,” said Servais. “He said, ‘Absolutely, you can do that.’ When I went out to do it, the other three umpires came in and said, ‘You can’t do that unless it’s a pinch-hitter.’ That’s why I went to the umpire first. It’s on me. I’ll take all accountability for it. [It] was certainly a change of momentum of the ballgame. My job is to put our guys in the best position to have success. I wasn’t able to do that tonight. I feel horrible about it. Tough spot to put George in, and Sergio.”

“I’ve never seen it before, but it is what it is,” said Kirby, who took the loss in his third career start. The rookie -- ranked by MLB Pipeline as the Mariners' No. 2 prospect -- went five-plus innings, allowing five runs on eight hits and one walk.

“I just didn’t really get ahead a lot,” Kirby explained. “It kind of put me in bad situations and counts. I could have used my pitches more, for sure. Getting ahead was the biggest thing.”

“George wasn’t as sharp as he’s been earlier, the first two times out,” said Servais. “Certainly, Trevor Story was on him tonight.”

Story -- a player the Mariners targeted during the offseason -- had his best game in a Red Sox uniform, belting three home runs and driving in seven.

The Mariners took the lead with four runs in the second, with three coming on Dylan Moore's second home run of the season. But after running Red Sox starter Rich Hill from the game after two innings, they were unable to solve Boston’s bullpen until the game was out of reach.