Report: AJ McCarron has torn meniscus
As other's have said, this is likely no big deal unless he goes out and gets surgery. More than likely he will just take some meds, do some stretching, wear a brace, and get it taken care of at the end of the season.
I tore a meniscus in my knee and it hurt pretty badly, but I was running around on it just a few days later with no issues (was swollen as frick and hurt a good bit, but if I could handle it then I'm willing to bet a D1 QB can take the pain). I did some brief rehab and stretches for a few months and never even had surgery and I'm fine now. Every now and then it will act up, but 95% of the time I'm pain free.