Peterson has what is known as "bucket handle tear." I'm told that's a minimum of 3-4 months out and up 6 months usually.
This is not a trim, but a full repair, which is why he's going to be out so long. Jerick McKinnon is likely to start with a healthy dose of Matt Asiata on third down (due to his superior pass protection), around the goal line and late in games, as he serves as the team's closer. (Asiata saw seven touches while McKinnon saw only two with the Vikings nursing a lead in Week 2.) Both players are good additions this week, though the Vikings' running game has yet to have much success this year. For a breakdown about how McKinnon and Asiata split the work with Peterson sidelined in 2014, be sure to check out John Paulsen's article on the subject.