According to Mike Monroe of Spurs Nation, this is Leonard's explanation of what happened:
I just hit the hardwood on one of those nails, really hard,” said Leonard, who made two of the biggest plays of the period for the Spurs. It busted my knee wide open.
It kind of felt funny when I tried to explode up the court when we got the ball and I tried to cut to the basket, so I had to come out and see what was wrong before I injured it worse.
Although I'm certainly not a person who would mock anyone who just suffered an injury, this type of cause for a knee injury is definitely out of the ordinary.
We're not entirely sure what Leonard means when he says that he "busted [his] knee wide open". It could mean that the nail he landed on tore a ligament in his knee, but it didn't look like it was a major injury since he was seen walking to the bench by himself.
For all that we know, there may be some information on the status of his knee that hasn't been revealed yet by the Spurs organization.
Honestly speaking, how can a nail be sticking out on the hardwood in the middle of a nationally televised NBA game without anybody knowing? The NBA courts are some of the most impeccable and well-cared for playing arenas in all of sports, so how is it possible that the employees in charge of it missed something like this?
This would definitely bring up some questions and concerns regarding the safety of the environment, especially during a professional contact sport like basketball.
Had Leonard (or any other player for that matter), landed on the nail with his face and incidentally got hit on the eye, the injury could even be life-threatening.