Ole's problem was that he was too much of a "friend" to the players and less of a "boss". They didn't fear him and as such standards began to fall when they realized they needn't try that much and can instead coast by. They were still disappointed by his firing, after all - they liked him, but they didn't fear him. SAF understood that the perfect balance of being a good manager is to be ruthless and at the same time caring to your players. He'd smack you with a shoe in one moment when you do something wrong and the next caress you in his arms when you do something right. This instills absolutely unwavering loyalty and every player gives it all, even those that were not quite good overperformed greatly. He was a master of psychology. In contrast to Ole, Mourinho by the time he came to United was on the opposite spectrum - he was overly harsh and despite being right, didn't have the necessary diplomatic skills not to engage in constant conflicts with both management and players.
Let's hope Erik Ten Haag possesses even the smallest of bit of that talent SAF had because it'd be more than enough to get us back to the top