Anything of course is possible in club football. And with Rooney's staggering salary coupled with the fact that Ferguson is still upset about Rooney holding out a couple of years ago, I wouldn't be surprised to see him gone. If so however, United should have sold him last year when they would have gotten twice as much as they'll get this coming off-season. Not convinced Welbeck or Hernandez are suitable replacements though. Even with the fitness and commitment issues, I still think United would be crazy to get rid of him.
I'm not sure I see Rooney being a regular in top tier teams, such as Manchester City, Bayern, Madrid or Barcelona; they all just have too much talent to ensure regular playing time. There are few other teams that could afford his wage demands and I don't really see him playing for a 2nd tier club. So in effect, Rooney needs to be careful how he plays this, if United hasn't already made the decision.
If they do sell Rooney though, I don't think Ferguson will necessarily go after anyone significant. I think he see's Welbeck and Hernandez as serviceable with Van Pierse as the real "replacement". And don't forget they just signed Kagawa who can play in that deep role that Rooney plays best just behind the single striker. So although they may sign a marginal or younger replacement, they don't a one for one swap that can start from day 1.