I've seen in-game injuries in three of the four games I've played so far. Much like in real-life, they were only tweaks so the players were able to return. I imagine as I play more games (like try a season instead of "play now"), some of those injuries will last longer.
I saw one in a playoff game where i (Dallas Mavericks) were playing the LA Lakers, Kobre bryant hurt his Left ankle. But he was still able to come back in the 4th (DAMN! lol)
Since I've been playing the PC version I haven't had or seen any in game injuries. On the PS2 version I saw a lot more. I don't know if anyone else has noticed this, or if it's just a coincidence. I have my injury frequency slider set to 60.