Manchester United have one of the most successful managers in the modern game. They broke the world record transfer fee for a player. They spent in excess of 120 million in the summer. They have the brightest young talents in the game. They have 2016s top goal scorer, world cup winners, International team record breakers. Premier League winners.
Last season Van Gaal, another highly decorated manager, finished 5th and won the FA Cup with a far weaker team. He was sacked. United may well be on course for another domestic cup.
However, despite an unbeaten run of 13 games! They remain 6th place in the premier league, 10 points from top of the league. They were also only runners up in the Europa League group stage.
If that doesn't show you the gravity of a Premier League manager's job nothing will.
I don't even need to bring in discussions about Koeman's time with us in comparison because I think everything that needed to be stated in relation to this point is stated so above.