Sorry, but there's no chance United could sell Welbeck and Hernandez and not get any sort of replacement in. And for the amount they'd have to pay to get a decent 2nd/3rd choice striker, they may as well keep Hernandez, a player who can get the odd goal but is also content to be a backup player.
If United bought a top-drawer forward, like Cavani, there's no way LvG could keep everyone playing and happy.
Hernandez is pretty much perfect for United. Welbeck, on the other hand, can go as far as I'm concerned. He's not quite as happy to play backup and so United have tried to squeeze him into the team in all sorts of positions, none of which he does well at. Let Young, Valencia, Shaw, Rafael play the wide positions, and Rooney, RvP and Hernandez play up front. Mata and Kagawa can cover the attacking mid role, and then there's still Januzaj to provide cover. A top quality wide player, like Cuadrado, wouldn't go amiss though...