ST. LOUIS — The Boston Red Sox face yet another significant setback in their rebuilding efforts, as first baseman Triston Casas has been shut down from all baseball activities due to a left-side intercostal strain, with the club now bracing for an extended absence.

Manager Alex Cora confirmed Saturday that doctors are still reviewing Casas’ latest MRI results, but the early indications point to an oblique-related issue rather than a recurrence of the torn rib cartilage that sidelined him for 63 games two seasons ago. While the team is awaiting a final diagnosis and a second opinion expected early next week, one thing is already clear: Casas will not be returning to the lineup anytime soon.

“Right now, we’re going to treat it like an oblique, rib injury,” Cora said. “He’s going to be out for a while but we’re still waiting for the news from the doctor.”

The development marks a frustrating chapter in what has been a prolonged recovery journey for the 26-year-old slugger. Casas suffered a ruptured patellar tendon in his left knee on May 2 last season, underwent surgery, and missed the remainder of the year. He made strong progress during the offseason, taking part in batting practice and intrasquad games throughout spring training. When the Red Sox broke camp and headed north, Casas remained in Fort Myers with plans to ramp up in extended spring games in early April.

His knee continued to strengthen, but progress stalled in the first week of April when he experienced pain on his left side. An attempt to resume swinging off a tee on Monday brought enough discomfort to prompt the team to shut him down completely for further evaluation.

According to a source, those close to the situation believe a worst-case scenario has been avoided. Further details on the exact severity of the oblique issue are anticipated Monday or Tuesday following another medical opinion.

“From what I know right now, our doctor feels like this has nothing to do with what happened before,” Cora added. “But let’s wait for the surgeon, see what he has to say and go from there. As of now, they’re going to treat it like an oblique/rib kind of thing.”

The injury couldn’t come at a more inopportune time for the Red Sox, who have been counting on Casas to anchor the middle of the lineup and provide power from the left side. His combination of plate discipline and raw strength made him one of the organization’s most important young building blocks before the string of injuries began. Now, Boston must navigate the early portion of the season without one of its key offensive pieces while continuing to monitor his long-term recovery.

For now, the focus remains on caution and thorough evaluation. The Red Sox will proceed carefully with Casas’ rehabilitation, determined to avoid any further complications as they await clarity on a timeline that remains uncertain.

As the organization digests this latest blow, the hope is that once fully healed, Casas can finally deliver on the immense potential that made him such a highly regarded prospect. Until then, Boston’s lineup will have to find production from elsewhere while the club’s medical staff works to get their young first baseman back on track.