It is too late for this fight. I actually think GSP had a great chance before his injury and the fight would have been GSP taking Silva down at some point in each round with Silva landing some serious ammunition before hand. Makes no sense at this point in thier careers or at least not at the moment. Maybe if GSP gets his sharpness back.
It has to happen now, but I can't predict the future. If it were to happen next year, I believe GSP will hold some serious advantage going into the fight. This should be a title fight, but I don't see Anderson Silva's belt on the line.