CLEVELAND -- Another day, another injury for the Red Sox, who have become almost numb to it at this point.

The latest development occurred a few hours before Wednesday night’s game against the Guardians, when righty Brayan Bello was placed on the 15-day injured list retroactive to April 21 with right lat tightness.

Three-fifths of Boston’s Opening Day rotation is on the injured list, as Bello joins Nick Pivetta (right elbow flexor strain) and Garrett Whitlcock (left oblique strain) on the shelf. And that’s not even counting Lucas Giolito, who is missing the entire season following right UCL surgery.

“Obviously we’re down three important [starters], four actually,” said Red Sox manager Alex Cora. “I don't even know how many, but we’ve just got to continue. The good thing is that we’ve been playing good baseball.”

Bello was scheduled to start Thursday’s series finale against the Guardians, but Cora will likely go with a bullpen game instead. Righty reliever Zack Kelly was called up from Triple-A Worcester to take Bello’s spot on the roster.

At this point, the Sox are hopeful Bello can pitch when he is first eligible, which would be May 7 in Atlanta.

“He'll be OK,” said Cora. “This is more about taking care of him. Hopefully he'll be back when he's supposed to come back. He's very important for this organization, not only now, but in the future, and I think we take care of this and he'll be fine. Obviously, there's no guarantees. But from my end, I'm not that concerned.”

How hard is it to deal with so many pitching injuries at once?

“It's not hard at all. Somebody else has to step up,” Cora said. “That’s the nature of this business. It’s 162-plus and you’re going to go through stuff like this. The good thing is that Nick is trending in the right direction. I do believe Bello is going to be back with us sooner rather than later, and Whitlock is feeling better. We’ve just got to continue to grind and keep trying to win games.”

Bello said he first experienced discomfort in his right side in his start at Fenway Park against the Angels on April 14. Five days later in Pittsburgh, Bello turned in his best start of the season, firing six scoreless innings while allowing one hit and striking out seven.

“I don’t think it’s that serious at all,” said Bello. “It’s just tightness. I can still throw. [We’re just being] very cautious. I’m going to be ready to throw six, seven innings when I come back.”

After signing a six-year contract extension in the weeks leading up to the regular season, Bello had registered a 3.04 ERA and 26 strikeouts through his first five starts, spanning 26 2/3 innings.

But the Red Sox’s injury woes have not been confined to their pitching staff. They lost shortstop Trevor Story for the season earlier this month due to a shoulder injury. Slugging first baseman Triston Casas was also diagnosed with a left rib fracture earlier this week and will be “out for a while,” according to Cora.

Vaughn Grissom, the club’s starting second baseman, has yet to play for his new club as he recovers from a right hamstring injury he suffered in Spring Training. Grissom is on a Minor League rehab assignment and should return soon.

Tyler O’Neill, the team’s best hitter the first two weeks of the season, returned from a week-long absence on the concussion-related injury list on Tuesday.

Rafael Devers, the team’s best hitter, missed 11 of the team’s first 24 games due to a left shoulder injury and then a left knee injury. After missing five straight games due to the latter ailment, Devers was back in Boston’s lineup as the DH on Wednesday.