Van Gaal's 14/15 clear out was pretty huge, to be fair.
Of just the senior players; Rio Ferdinand, Nemanja Vidic, Alex Buttner, Patrice Evra, Danny Welbeck, Shinji Kagawa, Tom Cleverley, Nani, Javier Hernandez, Darren Fletcher, Wilfried Zaha, Anderson, Nick Powell and Michael Keane were all either released, sold or loaned out never to return (Hernandez came back briefly a year later, but was gone again).
You could go a bit deeper and even include guys like Bebe, Angelo Henriquez, Sam Johnstone (still out on loan, for the 100th time), Ben Amos, Will Keane, Tom Lawrence, and a good few more reserves who were axed that year.
Brutal clear out.
This one won't be as massive, but I think Mourinho will be quite ruthless, and I predict some fans will be upset.