Nothing brings out scare stories amongst football fans better than a player tearing his ACL. You will hear everything, from his career is over, to him not ever being the same again. It's a bad injury for sure, but nowhere near as bad as made out.
Xavi himself, tore his and was back within 5 months. Totti tore his and was back in 4. Depends on the tear and we will find out the extent of Bernie's later on today.
As pointed out above by Messi983, your only looking at big problems if you damage the meniscus. I suspect the chances of him having done so are very small as he would have been in much more pain.
The worse case here is looking at him being out all season. But he can come back next season and be as good as gold. This injury should not impair on his upward trajectory or potential ceiling as a world class footballer.