We had conflicting reports earlier on about how severe the injury to Adrian Peterson's left knee was from a few different sources, but it appears that things are starting to solidify. And, since this is the Minnesota Vikings, you can safely assume that the worst-case scenario is the one that we're looking at.
Peterson has, indeed, torn two of the three main ligaments in his left knee, the anterior cruciate ligament (ACL) and the medial collateral ligament (MCL).
It is unknown how long the rehabilitation period for something like this is going to be for the Vikings' running back, but there is a very good chance that he will have to start the 2012 season on the Physically Unable to Perform (PUP) list, meaning that he would be ineligible to play the first six weeks of the 2012 season, but would still be able to practice with the team in order to get himself healthy. Peterson would then give it a go next October.
Obviously, that's all speculative at this point, but this is about as bad as the Minnesota Vikings could have expected for the man that is, currently, the face of their franchise.