WALTHAM, Mass - The Boston Celtics took a massive hit to their frontcourt depth Monday in learning that rookie big man Daniel Theis has suffered a season-ending meniscus tear in his left knee. The injury occurred during the final minute of Sunday night's game against Indiana, when Theis' knee buckled while trying to defend Pacers guard Victor Oladipo.

Celtics coach Brad Stevens was greatly disappointed to hear the news Monday morning, but he anticipates that Theis will be able to push through the rehabilitation process and eventually come back stronger than ever.

"I appreciate the way that he approached every day, and he'll approach the rehab the same way," said Stevens. "But you really feel for him. He was a guy that came in knowing he could play, but maybe not even expecting to have the type of season he had. And he didn't change that when we brought Greg (Monroe) in and mixed up minutes here and there. He just kept being who he is, and we're going to miss him on the court. But he'll have a full recovery and he'll be able to get back better than ever when he steps back out on the court."

Despite the injury, Terry Rozier says that Theis was in good spirits Monday morning when he came into the team's training facility. As locker neighbors, the pair have grown particularly close this season, so Rozier is going to miss Theis' company on and off the court.

"He's fine. He's not down," said Rozier, relaying the interaction he had with Theis. "That's just the type of guy he is. I felt like I've known him for a long time and he's just a great guy. We hang out on the road a lot, so it's just tough to see it. All we can hope for is a speedy recovery."

With Theis' season now over, the Celtics are trying to figure out how to best to fill his void. Fortunately, the C's have some depth in their front court, but they'll also have to get a bit creative in order to make up for Theis' defensive versatility against small-ball lineups.

"One of the reasons we wanted to sign Greg was because you never know what can happen with your depth inside," said Stevens. "But then there have been times where we've played smaller with Theis at the 5, and he's been able to switch and do some of those types of things. So, you might see Semi (Ojeleye) out there a little bit, kind of á la the Summer League stints where he guarded more of the skilled 5s and was able to move his feet on switches and really use his athleticism."

Guerschon Yabusele will also be joining the C's following a stint with the Maine Red Claws, and Stevens added that the rookie big man "will fill a role."

The C's will have more roles to fill than just Theis' for the foreseeable future, as they are battling a number of injuries to key players. Kyrie Irving could miss some time with left knee soreness, Jaylen Brown is sidelined with a concussion, Al Horford is home sick, Marcus Smart is undergoing tests for a thumb injury, and Shane Larkin is still facing a minutes restriction because of soreness in his right knee. And, of course, Gordon Hayward remains out indefinitely as he rehabs his left tibia/ankle.

Because of the plethora of injuries the Celtics are dealing with, the team was forced to have a very light practice Monday afternoon. Stevens took the opportunity to bring all of the young players together and deliver a message that would lift their spirits during the time of hardship.

"Through all the things that are going on, he was just making sure that everybody still believes in each other and that the team can still have success," Rozier said, relaying Stevens' message to the media. "Brad's great at that - at making everybody feel special and making everybody feel as one."

Stevens wanted his players to believe in themselves, because he wholeheartedly believes in them.

"We are equipped, and we will battle on," said the coach.

The battle will continue Wednesday night, when the C's host the Washington Wizards at TD Garden.