CHICAGO -- Brayan Bello watched the fly ball off of Gavin Sheets’ bat carry to deep right field. As the drive cleared the fence, Bello slammed his glove on to the mound in a moment of frustration.

Sheets’ go-ahead grand slam punctuated a frustrating fifth inning for Bello and the Red Sox, in Saturday’s 6-1 loss to the White Sox at Guaranteed Rate Field.

“This is definitely probably one of my more frustrating starts [in the Majors],” said Bello, who appeared to be taking the outing hard postgame, through an interpreter. “A lot of difficult things have happened. But it’s the big leagues, and you gotta move on.”

Bello got off to a strong start this season, carrying a 3.04 ERA through his first five outings. He went on the 15-day injured list on April 24 due to right lat tightness and returned on May 12. In six starts since, Bello has a 6.25 ERA in 31 2/3 innings.

“I really liked the way I started the season,” Bello said. “I felt like all my pitches were in a good place. I was executing well. But after that IL stint, things haven't been going the way that I want them to. I've had a lot of highs and lows.”

Red Sox manager Alex Cora noted pregame Saturday that Bello had one bad inning in two of his previous three starts. On May 22 against the Rays, he allowed three runs in six innings. They all came across in the second. On May 28 against the Orioles, he allowed three runs in five-plus innings. Those three came across in the first.

Bello allowed five runs on nine hits and three walks on Saturday, with just two strikeouts.

“In this one, the stuff wasn't as great,” Cora said postgame. “Velo was a little bit down, command was off. Just go back to the drawing board, help him through this and be ready for the next one.”

Bello was cruising early on Saturday, needing just 59 pitches to complete four scoreless innings. Even after he surrendered a one-out double to Danny Mendick in the fifth, the 25-year-old responded by getting Martín Maldonado to fly out to center fielder Ceddanne Rafaela.

That turned the lineup over to leadoff man Corey Julks. After getting ahead 0-1, Bello threw four straight balls to Julks: two sinkers and two sliders, all off the outer half of the plate, putting Julks on first. Bello then walked Zach DeLoach in a six-pitch sequence.

“I just didn't attack them how I should have,” Bello said.

Andrew Vaughn reached on an RBI infield single following the consecutive free passes, on a grounder Jamie Westbrook knocked down diving behind second base. It set the table for Sheets, who took a slider on the outside corner for strike one and then connected on an 88.1 mph changeup over the plate for the grand slam.

Bello has thrown his changeup second-most frequently the past two seasons, behind only his sinker. In 2023, opponents had a .196 average, .291 slugging percentage and 38.7 percent whiff rate against it.

This season, entering Saturday, opponents have hit .127 with a .310 slugging percentage and 35 percent whiff rate against his changeup.

“It’s definitely one of my better pitches,” Bello said. “It's one that I'm always working on, incorporating into the outing. It hasn't been working lately off of sliders that I have thrown. But I'm gonna keep working on it, because I need that pitch.”

Cora called Bello’s glove-slam “out of character” for the right-hander, but noted it came in the heat of the moment. The Red Sox manager equated it to a managerial ejection; Cora was tossed after Westbrook was called out on strikes on a borderline pitch to end the top of the fifth. (Cora acknowledged he was wrong and it was a strike, after he saw a replay.)

As much as the fifth inning got away from Boston, the Red Sox also finished 1-for-11 with runners in scoring position on Saturday. And for Bello, his next start offers a clean slate to get back on track.

“As I always say, man, it's a work in progress,” Cora said. “This kid has electric stuff, but he's still learning at the big league level. And the more he pitches, the better he's going to be. We’ve just got to help him.

“My expectation out of him is to go out there every five days and give us a chance to win. He did that the last three [starts]. Tonight, he didn't do it. Be ready for the next one.”