They were undercover, he ran, they got pissed, and took it out on his face. When are they going to either get rid of these undercovers or use them correctly in conjunction with uniformed officers?
If NYC cops did not have a loooong history of abusing the public, they'd get the benefit of the doubt. The FACT is that the friendly-neighborhood policeman of myth has been turned into a commando that is now charged with going after people to "protect us" from "urban terrorists" like this kid.
Now, using your judgment, what's the plausibility of falling on your face and severely damaging only your eye? Wouldn't you think that your nose and/or your cheek would also show injury? It's not hard to visualize.