Uruguay left back Alvaro Pereira has taken a knee straight to the temple, appeared to be knocked unconscious and was stretchered off the pitch. But just one minute later, he re-entered the game, and he's still playing.
It's impossible to tell if Pereira is actually concussed without examining him, but it would be quite surprising if he isn't. The knee that he took from Raheem Sterling was nasty, and he definitely was not able to walk off the pitch under his own power. Also, just seconds after coming on, he headed the ball clear and made a very hard defensive challenge.
Also worth noting is how thin Uruguay are in defense. Their first choice defense isn't great, and they're currently down two players -- Diego Lugano is injured and Maxi Pereira is suspended. If Pereira can't continue, Uruguay will have to move a midfielder to defense or bring on Jorge Fucile, who simply isn't very good.