Closer Koji Uehara had pitched the previous two days, so manager John Farrell went to one of his trusted setup men instead. It didn't work.
"Didn't want to go three days in a row [with Uehara], day game after night game, the travel, all things considered," said Farrell. "Both Koji and [Junichi Tazawa] were the two guys we just tried to stay away from today."
And that gave Mujica a chance to come up big for his new team, but instead, he faltered.
Khris Davis started the rally with a leadoff double into the corner in left. Scooter Gennett dropped down a bunt, and Mujica went for the out at third. But Middlebrooks couldn't apply the tag in time, and everyone was safe.
"He was just a little bit up," catcher A.J. Pierzynski said of Mujica. "They got the leadoff double. We tried to be aggressive, get the lead runner on the bunt. It kind of led to a big inning instead of just getting a guy out. But those things happen. I thought when he bunted it, he bunted it hard enough that we could get the guy at third. We tried to be aggressive. Unfortunately, it was a real close play and he was safe."
It looked like Davis might have been tagged out when he over-slid the bag, but replays weren't conclusive enough for the Red Sox to think a challenge was worthwhile.
"I felt like he kind of came off the bag a little bit, but that's hard to see," said Middlebrooks. "I wasn't sure at the time, because it happened so fast. But after I watched the replay, I thought I might have had him. That happened so fast, nobody could see that."
And the near miss wound up being costly.
Lyle Overbay unloaded for a double to right and the Brewers went ahead for good.
This, after Overbay watched the ring ceremony for a club he was nearly a part of last season, only to be cut at the end of Spring Training.
"It wasn't 'payback,' because I didn't have any bad feelings, but it's always nice to take the World Series champs down," Overbay said.
Mujica wasn't even able to keep his team within striking distance. Carlos Gomez and Aramis Ramirez added RBI singles to make it 6-2.
The line on Mujica? Two-thirds of an inning, four hits and four runs.
"Well, they came out swinging against Mujica," said Farrell. "Knowing that he's been in the National League Central, they're familiar with him. At the same time, Edward has had a lot of success against that lineup. He mislocated a couple of balls, and unfortunately, today, paid for it."
Jake Peavy, who had the honor of getting the ball in the home opener, rose to the occasion. The righty gave the Red Sox six strong innings, scattering six hits and allowing two runs. He walked two and struck out four.
"Anytime you have a chance to win and you don't have your best stuff, you don't feel the best or have the best feel, you're OK with it, but you expect more," said Peavy. "At the end of the day, you want to pitch deep in the ballgame, you want to give up less than two runs. That second inning got away from me. I couldn't stretch it out late. We'll get back at 'em tomorrow."
The Brewers broke out first, getting a solo homer from Jonathan Lucroy to open the top of the second. With one out, Davis drilled a double to center and Gomez came through with an infield single to make it 2-0.
Back came Boston in the bottom of the second. Mike Napoli led off with a walk. Grady Sizemore ripped a single to right and Napoli was able to score when Logan Schafer made a throwing error to third.
Middlebrooks tied the game with one swing in the bottom of the third, lofting a solo homer over the Green Monster.
"It felt good," said Middlebrooks. "I mean, it was a big day for us with the ring ceremony and all that and everybody having family in. It was an emotional day for everybody who was a part of that last year. It felt good. It always feels good to hit a homer, but in that situation, it felt even better."
The only thing lacking from a memorable day at Fenway was a win by the home team.
"We were ready to play," said Middlebrooks. "Their starter kept us off-balance, especially in big situations with guys in scoring position. He had a good changeup and got a few guys out front. Just get them tomorrow."