Sadly enough, this could be the end for Green much like a number of QBs before him. I can think of Aikman, Young, and Humphries just off the top of my head. I don't know how serious this concussion is, but I know that the more concussions you get, the easier they happen again, and the longer they tend to linger. Football is a violent sport, and Green has gotten a lot of mileage out of his career.