As reported by SBNation's Celtics Blog, Garnett suffered the injury late in the first quarter of the game after going up for a successful dunk. He tried to stay on the court, but ended up falling in obvious pain. Garnett was able to make it to the locker room, where he received x-rays. The initial scan indicated no fracture. Garnett will have an MRI on Thursday to diagnose the injury further.
ESPNBoston's Chris Forsberg pointed out that a post-Christmas injury Garnett is becoming a yearly occurance. Last December, he hyperextended his right knee, which he has had surgery on in the past. He missed ten games with that injury.