Add Triston Casas to the growing list of key Red Sox players sidelined due to injury.

The Red Sox announced Sunday morning that they’ve placed the talented first baseman on the 10-day injured list with a left rib strain. Catcher Tyler Heineman has been called up from Triple-A to fill his spot on the big league roster.

Casas left Saturday’s game after one at bat with what was described at the time as left rib discomfort. Following the game Red Sox manager Alex Cora expressed concern for Casas, saying “no, he’s not doing ok” when asked and adding that Casas had told him he could probably go out and field his position but that he’d need someone to hit for him his next at bat. Casas had already left the ballpark by the time the game ended and was still undergoing tests when reporters were in the clubhouse.

With is lineup already decimated by injury Casas is someone the Red Sox can’t afford to be without. The 24-year-old has gotten off to an excellent start to his second full season as a starter and is currently batting .244 with six home runs, 10 RBI and an .857 OPS.

The Red Sox are also without shortstop Trevor Story (fractured shoulder, out for the season), second baseman Vaughn Grissom (left hamstring strain, has yet to debut), outfielder Tyler O’Neill (concussion, 7-day IL) and Rafael Devers, who isn’t on the IL but has missed nine of Boston’s first 22 games due to shoulder and knee issues.

The club’s 40-man position player depth is also almost completely exhausted, as Heineman was the last healthy position player on the 40-man still available in Triple-A.