CLEVELAND -- Boston Red Sox first baseman Triston Casas is out indefinitely after breaking a rib on his left side while taking a hard swing in Saturday's game at Pittsburgh.

Casas, who had been placed on the injury list with a strain, underwent tests in Boston that revealed the fracture. Manager Alex Cora said there is no timetable on when the 24-year-old will be back.

"He'll be out for a while," Cora said Tuesday before the Red Sox opened a three-game series at Progressive Field against the Guardians. "We just have to be patient. It has to heal on its own, so we just have to be patient."

Cora expects Casas, who was batting .244 with six homers and 10 RBI, to return at some point this season. Cora said Casas told him that his rib was bothering him last week in a series against the Guardians.

On Saturday, Casas injured himself while fouling off a two-strike pitch in the first inning. He drew a walk and was replaced defensively in the next half inning.

"He's still in a lot of pain and it's difficult to rest that way," said Cora, who texted with Casas earlier in the day. "Obviously, he's disappointed, but it is what it is. There's nothing we can do, just got to be patient."

Cora said Boston's lineup will miss Casas' presence.

"He's a force," Cora said. "We saw that last year in the second part of the season. He was really good and then he got hurt. He's very important, but we've got to keep going, man, and we're not going to stop."

With Casas out, Cora said Bobby Dalbec will get most of the playing time at first. Dalbec has been in a prolonged slump dating to last season and entered the series batting just .033.

He snapped a 0-for-19 drought on Friday with a double against the Pirates.

"We're going to play him a lot," Cora said of Dalbec. "Obviously, the front office is studying the situation and looking around to see if we go somewhere else for that. I talked to Bobby today. He's done if before. In '21 he was really good for us and hit the ball out of the ballpark and played good defense."

Casas' injury is just the latest for the Red Sox.

Outfielder Tyler O'Neill was re-instated on Tuesday after being in concussion protocol following a scary collision last week at Fenway Park with third baseman Rafael Devers. O'Neill sustained a gash on his head that required eight stitches.

Devers has been slowed by a bone bruise in his knee and hasn't played since April 17. Cora decided not play him against the Guardians because of wet field conditions.

Also, the Red Sox placed catcher Tyler Heineman on the 10-day injured list with a right hamstring strain.

Boston's rotation has been ravaged by injuries. Garrett Whitlock was placed on the 15-day injured list last week with an oblique strain. Nick Pivetta also is on the 15-day IL with a right elbow flexor strain.