Either Fellaini doesn't play or you get used to United being a limited team who wont be at the level they want to be.
It's that simple really. He can try, he can have decent games, but he can't change the fact he's a very limited midfielder who lacks mobility. Him being in the team creates problems that can't be resolved by changing who you put around him.
You can't get away with covering less ground than every team you play. You can't get away with moving the ball slower than an opposition can regroup. These are problems that create limitations that are impossible to compensate for during a game, so create a glass ceiling for the whole team.
People are obsessing over Rooney, but a team with Wayne Rooney in it can win a league title if the rest of the team is good enough. A team with Fellaini in it can not.