When it rains, it pours.

The injury-riddled Boston Celtics were dealt another tough blow Monday, learning that reserve big man Daniel Theis has a torn meniscus that will require season-ending surgery, according to Shams Charania of Yahoo Sports.

Theis suffered the injury Sunday night during Boston’s game against the Indiana Pacers, and head coach Brad Stevens initially didn’t believe it was too serious.

“He walked into the training room as I was leaving,” Stevens said after the Celtics’ 99-97 loss. “He said he felt a little better. I think he got kneed, but I don’t think that that’s anything more than a knee-to-knee or knee-to-thigh — right above his knee.”

But Theis underwent an MRI on Monday morning that revealed a far worse ailment, it appears.

The 25-year-old rookie from Germany made the most of his limited minutes this season, emerging as a surprising contributor off the Celtics’ bench. He averaged 5.3 points and 4.9 rebounds per game despite playing under 15 minutes per contest and shot 54.1 percent from the floor.

Kyrie Irving also left Sunday’s game with a knee injury, while Jaylen Brown (concussion) and Al Horford (illness) missed the contest, as well.

The C’s could lean more heavily on Greg Monroe in Theis’ absence, while fellow big men Aron Baynes and Marcus Morris also could see an uptick in minutes.

Thumbnail photo via Bob DeChiara/USA TODAY Sports Images