6:57 PM ET

MILWAUKEE — It will be a long time before Boston Red Sox fans know if the offseason signing of Japanese star Masataka Yoshida will pan out. Suffice to say, after his eighth-inning outburst Sunday, the outlook looks a great deal rosier.

Yoshida hit a go-ahead solo homer in the eighth, then later in the same inning capped a nine-run Boston rally with a 407-foot grand slam to right as the Red Sox beat the Brewers 12-5 on Sunday.

That gave Boston a series win against the first-place Brewers after Milwaukee erased an early three-run lead and carried a 4-3 advantage into the eighth.

“This is a team — we’re going to grind until the game is over,” Red Sox manager Alex Cora said. “We’ve been doing this the whole season.”

Yoshida’s first home run came after Justin Turner knotted the score with his second homer of the season, giving the Red Sox back-to-back homers for the first time since 2021. Both blasts came off Milwaukee reliever Matt Bush.

The grand slam Yoshida rocketed into the right-field seats off of Javy Guerra left his bat at 105.4 mph, an emphatic statement for a player who entered the game mired in a season-long slump, albeit one that has shown signs of lifting in recent games. A career .327 hitter in Japan, Yoshida entered Sunday hitting just .213 with three extra-base hits in 71 plate appearances to start his Red Sox career.

“I think I feel more comfortable than before this series,” Yoshida said through a team interpreter. “I talked to the hitting coach about my mechanics and hitting form, and I found a better one.”

Apparently so. Yoshida became the first Red Sox hitter to hit two homers in an inning since David Ortiz, who did it against the Texas Rangers on Aug. 12, 2008. When that was mentioned to Yoshida, he knew he had joined some rarefied company.

“I’m really honored to hit the same as a Red Sox legend,” said Yoshida, who signed a five-year, $90 million contract with Boston during the winter meetings last December.

For now, Yoshida can put those early ugly numbers aside. He started the Boston scoring with a first-inning sacrifice fly, giving him six RBIs for the game and 15 on the season.

Even as Yoshida’s average remained under .200 entering the series, the Red Sox felt like the quality of his at-bats was trending in the right direction. He had five hits over his past three games before Sunday’s breakout.

“Obviously there’s an adjustment and there’s things that he knows he needs to do to hit the ball hard in the air,” Cora said. “He got two pitches today, and the second one — that was something to watch.”

At 29, and with a storied international career that includes a recent World Baseball Classic victory with Team Japan, Yoshida isn’t a traditional tenderfooted rookie. Still, when his grand slam left the yard at American Family Field, he became the first rookie to hit two homers in an inning since the Yankees’ Joe Pepitone on May 23, 1962.

Turner, who drew a walk in advance of Yoshida’s slam, quipped, “I told him I was going to do the same thing, but [Guerra] didn’t throw me any strikes. But I’m happy for him, obviously. He’s been grinding a little bit early and for him to take these quality at-bats this whole series has really been spectacular.”

For Yoshida, it’s been a matter of adjusting to a new team, a new country, a new style of baseball. After the game he spoke of trying to see opposing pitchers better by opening up his stance while working on his timing. The adjustments seem to be paying off.

Nevertheless, despite the spotlight performance, Yoshida isn’t ready to start celebrating just yet.

“This is a really good day for me,” Yoshida said. “But still, we are in the beginning of the season. I would like to keep preparing, keep playing hard all season.”