I don't care about (american) football, but I once watched a 60 minutes featuring this player and found him very likable. Here's hoping he can fully recover.
Repairing damaged knee cartilage or reconstructing ligament is, as far as I know (not much), a fairly common procedure these days in pro-sports medicine, via autologous transplantation or using artificial tissues. At least there are more than a few soccer players who went through those procedures, and I'd say that playing soccer is more demanding to the knee that playing QB (or maybe not, I don't know). I remember a particular case, with former Porto's captain Jorge Costa, who had this exact same problem, an infection after a surgery on the ACL, two different times (and in both knees), and a few years later was lifting the Champions League cup, so I'm quite puzzled how is this that threatening.