He got rid of Rafael, Nani, Welbeck, Evans, Keane, Januzaj, hell even Zaha. They may not all have been world beaters but it's a strong base to build a squad from, all of them were young and had very good seasons at United or would go onto be very good elsewhere.
Instead he sold them all and replaced them with crap like Bastian, Schniederlin, Depay and Darmian. The end result was that when Mourinho took over, we were many years behind City in terms of squad depth and ability and have been trying to play catch up filling 50 different holes since. We made two mistakes - not giving the job to Mourinho earlier, and hiring the Van Gaal and letting him do whatever he wants with the squad. It set us back 5 years.
Put it this way - had Sir Alex still been in charge in 2014, or had Mourinho taken over after Moyes, a lot of those players would've been used.