Koeman before he turned into a bellend.
I think that was kinda day 1. I know people loved the whole beating up on players in the press (ala Barkley) after the pandering era of Martinez but personally I think that's a no-no and one of the reasons a lot didn't warm to him. By all means say something isn't good enough when it isn't, or acknowledge a mistake but you don't need to single out players in the press when they really have to ability to reply, keep that stuff for behind closed doors. Plus if you want to sell them it doesn't do anything good to their value.
I don't recall support for a manager evaporating so quickly as it did under Koeman. I think his attitude with certain players and fans from the get go didn't help as there was little tolerance once the results went the wrong way.