You are right about Ten Hag making many mistakes in Manchester United. In fact, Ten Hag makes Sancho leave. In comparison, this player could help the team and Ten Hag made many mistakes in this team during the summer because the players they bought during the last summer didn't help Ten Hag and his team at all. However, still, we should not forget Ten Hag was the coach who helped Manchester United once he joined this team while they didn't have a good station at all.
Since the beginning of last season it was as if all his ideas for Manchester United was switched off by the pressure of EPL and at some point I thought ten Heg would use the same style he used with Ajax and apply them to the UTD team (by making them unstoppable), things went side ways for him. Getting some of the Ajax players seems to be the best according to him but they came to even make the team look like a team without any future plans and you see some of ten Heg's best playing absolutely bad and nothing positive to help the team and he has Sancho that he'll use as the 1st or 2nd key man after Rashford but chooses to loan him out which to me was the second mistake ETH made last season.
Is best to work with what you have at the club except the players ain't good enough before you start seeking for players from your old club, not by coming and start recruiting players any how. What ETH was trying to do can work for other coaches but he thought it will for him, but the window is still fresh to try get some better replacement to some of those players he bought last season.