"We got beat with our best guys out on the mound," manager Kevin Cash said. "It lined up perfectly. Sometimes that's going to happen."
The Rays like their chances for a win any time they head into the seventh with a lead. They can then have Kevin Jepsen pitch the seventh, McGee the eighth and Boxberger the ninth.
Jepsen had already logged a scoreless seventh before McGee ran into problems from the get-go in the eighth.
Pinch-hitter Rickie Weeks greeted McGee with a single, and Robinson Cano followed with a single to left. McGee's effort then took a gut punch when he got Nelson Cruz to ground to second base, but Nick Franklin could not handle the chance to convert what appeared to be a sure double play.
Instead of having two outs and a runner on, McGee faced Kyle Seager with no outs and the bases loaded. McGee threw a 94-mph fastball and Seager connected, sending a drive deep into the right-field seats for a grand slam that put the Mariners up, 6-3.
"I was just trying to get ahead right there," McGee said. "Especially with no outs and the bases loaded. I don't want to fall behind him. You know, he got a decent pitch. It leaked back over the plate, and he hit it out."
The home run snapped McGee's streak of 96 plate appearances against left-handed batters without allowing a home run. He allowed that Franklin's error "factored in a little bit."
"I'm not going to lie," he said. "But at the same time, I have to make my pitches, and in that situation, don't want to fall behind him. If I do I'm going to have to go across the plate."
After the Rays tied the score with a stirring three-run rally in the ninth, Cash sent Boxberger to the mound to start the 10th. After Cruz singled and was retired on a caught stealing, Boxberger got ahead of Seager, 0-2, but once again the Mariners slugger got the job done, hitting his second home run of the game.
"I was able to get ahead early and just made a bad pitch," said Boxberger, who saw his streak of 12 scoreless appearances snapped. "I pretty much threw it right down the middle, and he didn't miss it, that was for sure.
"I was trying to go up with it and just put it right down the middle and right into his swing. You know, I was hoping it stayed in. I was hoping he got under it a little bit. But he got enough of the ball to carry it out."
McGee and Boxberger have been a lockdown tandem since last season, so the way Tuesday's game ended didn't feel right.
"I think the overall game tonight was just a little weird," McGee said. "How all that stuff happened in the one game. We might not see it again all year."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.