When it comes to baseball, expect the unexpected:

Jackie Bradley Jr. winning ALCS MVP and almost single-handedly slugging the Red Sox into the World Series in 2018.

David Ortiz going on to become a three-time World Series champion and Hall of Famer after the Minnesota Twins released him.

Spencer Arrighetti having the best start of any Houston Astros arm this season.

That last one occurred on Saturday afternoon at Fenway Park, when the right-hander fanned 13 Red Sox batters, setting a career high, the season-high for his team’s pitching staff, and propelling the Astros to a 5-4 victory and series win.

“Mixed it up. Good fastball, good sweeper, good changeup,” Red Sox manager Alex Cora said of Arrighetti. “Did an outstanding job keeping us off balance.”

While the Boston bats tried to get to Arrighetti, the Astros clubbed four homers – three of them with two outs – to score their runs. Yordan Alvarez’s fourth multi-homer game of the season led the Astros offense, and prompted Cora to draw comparisons two of the most formidable American League East bats of the century.

“Just controls the strike zone, just hits for average, he’ll take his walks. For me, he’s up there with (Aaron) Judge, to be honest with you,” the Sox skipper said. “It seems like he likes hitting here. That wall kind of like, keeps him close, you know? He can shoot it the other way, and then he can do that. Closest thing to David, probably, in the game. Very, very, very similar to David Ortiz.”

The Red Sox relied on the long ball, too; in fact, it was all they could do during Arrighetti’s seven innings, which matched his season-high.

After not hitting a single home run in Friday night’s loss, they took the Astros starter deep twice: Danny Jansen and Masataka Yoshida led off the fifth and seventh innings, respectively, with first-pitch solo shots. Those were Boston’s only hits off the righty, who entered the day with a 5.33 ERA over 20 starts this season, worth minus-0.2 Wins Above Replacement, and had a no-hitter going until Jansen’s homer. Arrighetti also worked around David Hamilton’s two-out walk and team-leading 29th stolen base in the third.

With Nick Pivetta’s next start pushed back to arm fatigue, the Red Sox had planned to turn to Cooper Criswell. When they found out Friday morning that he’d tested positive for coronavirus and would need to spend at least seven days on the COVID-related injured list, they pivoted once again, to a bullpen game.

The original plan was for Josh Winckowski to face the Astros lineup once – at most three innings – Cora said before the game. But despite facing five batters in the first, Winckowski gave Boston three innings.

Other than Alvarez’s two-out solo homer, Winckowski never yielded. He masterfully induced weak contact, and backed up by excellent defense, set the Astros down 1-2-3 in the second and third. The righty capped off his outing by getting leadoff man Jose Altuve and No. 2 slugger Alex Bregman out for the second time.

Alvarez’s first blast wasn’t a matter of missing the spot.

“That’s just a really good swing by him,” said Winckowski. “From what I saw, it was like, outer-third, little low, so yeah, just really good swing by him. Was (a) decently good pitch.”

Brad Keller, Zack Kelly, and Chris Martin took on the remaining six innings. Keller led the way with four innings of work, but was charged with the loss as he surrendered three earned runs on six hits, walked two, and struck out six. Keller gave up a two-run go-ahead homer to Alvarez in the sixth and a solo blast to Bregman – who’d scored on Alvarez’s second homer – in the seventh. In the eighth, Zach Dezenzo hit his first career homer off Kelly.

But as soon as Arrighetti departed, the Red Sox pounced and made things interesting. Bryan Abreu yielded a one-out single to Ceddanne Rafaela, and a two-out single to Jarren Duran, who promptly re-tied Hamilton with his 29th steal. After going 1-for-12 with runners in scoring position the night before, the Red Sox finally capitalized on their best opportunity of the day. Wilyer Abreu lined one to right and pulled up at first base and watched his teammates score a pair to pull within one.

There would be no game-tying knock, as Yoshida popped out to strand Abreu, but for a moment it was a new ballgame. Kenley Jansen began warming, but Chris Martin took the mound for the top of the ninth and maintained that one-run margin.

After striking out with the bases loaded the night before, Rafael Devers was ready for a big moment. He greeted Josh Hader with a double – his 30th of the season – high off the Monster to lead off the bottom of the ninth.

His teammates, however, were not. As Devers watched from second base, Danny Jansen and pinch-hitters Rob Refsnyder and Romy Gonzalez went in order.

“We put pressure,” Cora said. “We went with our two best pinch-hitters. It’s part of the game. I’m more pleased with this one than the one yesterday. … We were one swing away from winning it.”

The Red Sox are 61-54, have lost three in a row, and after pulling back to a half-game out of the third American League Wild Card spot earlier in the week, are two games back again.