The best solution is to sign Pep Guardiola or Ancelotti, because if they failed to bring Manchester United glory, the fans will blame the players instead of the managers, so they can survive and build Manchester United for many seasons.
Whether Manchester United will be able to bring in either of these two people with the money we don't know how much money the Red Devils have at the moment, however it will definitely take a long time to develop players, even if one of the top coaches is brought in by the Red Devils then there is a big possibility The Old Trafford public will wait the next two years so that the desired game can come true, but if it is far from expectations then of course it will be a waste of money.