You will know. Its what did for Colin Bell, Paul Lake, and Erling's dad when Roy Keane did him. And in a big ball of karma Keane did his too. Shearer did his. Gascoine in that FA cup final. Loads of players but these days it is recoverable with top surgery and careful rehab. It did used to be pretty much the end but it can be overcome now and has been with success among lots of players. Its tough rehab. Basically that ligament is small but tough and is the main short piece that runs right through the middle of the knee, about an inch or so long only it cant be fixed once gone. They generally twine hamstring and thread it through the gap and attach it to the start of the calf as a replacement. And all that needs to settle and then strengthen to be one workable strong and flexible piece.