McDonald went in for a hit, lost his helmet and stayed down on the field. It looked like he took a shot to the head, but he came up holding his right knee. Trainers came out on the field and put his leg in an inflatable brace, and then he left on a cart.
Officially, the team called it a leg injury. All that's known at this time is that he isn't coming back to the game. It's a big loss for the Rams, who are already thin at safety.
You can see his leg flop at the end of the play here, on contact with one his teammates.
Update: It's a broken leg, according to reports.
Rams S TJ McDonald suffered a leg fracture that led to him being carted off, 2 sources say. He was a Jeff Fisher favorite, tough blow.— Ian Rapoport (@RapSheet) September 27, 2013