He went over on his bad ankle while putting the ball in the net. But played on after and was moving freely enough. He was taken off but he was only supposed to play an hour anyway according to Wenger.
But yeah, it is quite tragic. Can't see his career having a Van Persie style turnaround because his injuries seem related whereas Van Persie kept snapping different parts of the body. And there's also the fact that Van Persie clearly had a lot of talent and was scoring whenever he was fit, whereas Diaby at his best runs past a couple of players and does something stupid at the end of it.