BOSTON -- For a moment, all the frustration of the day for the Red Sox was gone on what was initially ruled a game-tying three-run homer from pinch-hitter Reese McGuire in the seventh inning.

If that felt too good to be true for the hearty souls who braved the chilly air to be at Fenway Park on Wednesday afternoon, it turned out it was.

The umpires convened and decided it was a foul ball on the wrong side of Pesky’s Pole. The moment symbolized the irritation from not only a 4-1 defeat in the series finale vs. the Pirates, but also a three-game sweep, turning what started as a potentially promising season-opening homestand into one that moved Boston’s record to 2-4.

“We actually kind of practice that in BP, where we try to keep the ball fair there down by the foul pole and see if you can keep your backspin true so it doesn’t fade like that,” said McGuire. “When I made contact, I thought it was a tie ballgame, and it took a hard right turn at some point. It was definitely the highest of high excitement right there in the moment, and then it’s like, got to battle with two strikes.”

McGuire struck out to end the seventh inning with Boston still trailing by three.

What really came back to haunt the Red Sox on getaway day were two costly fielding mistakes in the top of the seventh inning, when the Pirates plated two insurance runs.

With a runner on second and no outs, Oneil Cruz hit a hard chopper to first baseman Triston Casas. The play for Casas was to step on first and take the out. Instead, he fired across the diamond, and Jason Delay beat the throw without sliding.

“If it's a one-hopper to him, oh yeah, he has it at third, but it wasn’t,” said Red Sox manager Alex Cora.

Casas said he was anticipating the ball being hit to him and firing to third, because he felt his team needed a big play. What threw him off was the way the ball was hit.

“I think with the way it was chopped, it kind of moved my eyes a little bit from being able to have a feel for where that runner was, going from second to third,” Casas said. “I guess I didn't have a feel for where he was and should have tried to give it to [reliever Kaleb] Ort or tried to beat Cruz to the bag, looking back on it."

Then came the next mistake -- actually, two on the same play.

Bryan Reynolds lofted a fly ball to medium depth in left-center field. Given that Masataka Yoshida doesn’t have the strongest arm, it looked like a sacrifice fly off the bat.

Mistake No. 1: Yoshida should have thrown to second to prevent the runner from tagging up from first.

Mistake No. 2: Once Yoshida threw home, Rafael Devers needed to cut the throw off. Devers let the ball go through, and it went to the backstop. He then had to scramble to get back to third base to take the throw there, but he was too late and Cruz was safe after a collision.

Devers acknowledged after the game that catcher Connor Wong yelled “cut," but the third baseman said he didn’t hear his teammate over the crowd noise.

“We made two bad decisions,” said Cora.

Cora was asked about another key decision: lifting Corey Kluber. He made only 67 pitches over five strong innings, allowing three hits and one run in the best performance by a Boston starting pitcher through the first six games.

“We’re good right there,” said Cora. “We feel like that was good enough, where we were with the bullpen, bringing in [John] Schreiber. They scored one and they kept adding on, but that was good enough for us [from Kluber]. He did his job. Reynolds was coming up, and we have one of our best relievers coming up in that spot.”

If the Red Sox had scored like they did in the first four games, pounding out 33 total runs, that line of questioning to the manager wouldn’t have even come up. But Boston’s bats went cold, starting in the second inning of Monday’s opener against the Pirates. The Sox scored three runs in the final 26 innings of the series sweep.

“When you are not scoring runs like the last two days, you’ve got to be on point,” said Cora. “Even when you score runs, you’ve got to play better defense. And I think we just make bad decisions. Overall, they kicked our butt. They came to Fenway Park, and they swept us.”