If the right kind of players are assembled, Erik Ten Hag will definitely do well with the lads and the club. It's okay to judge him but make sure use his first season or last season to judge him alone. He was brilliant in his first season because he had squad depth. I don't know why it hasn't been that way for them though.
Manchester United will have to invest in the defense and the defensive midfied role before any other thing else. Bad defense lead to all these golas scored a d conceded, if they're sorted out fully before the season kicks off, Manchester United will have a god backline. The name Victor Lindelof is also open to leaving Manchester United as he is being reported to be joining Jose Mourinho in the Turkish League Competition.
They will do better and Erik Ten Hag will not be sacked. If these two areas are sorted them they can now go on to another position to buy!
With Ten Hag retained, I hope that Manchester United can recover and compete fiercely again like in its era, because it has been a long wait for Manchester United to return with its famous performance in that era. It is also impossible for Manchester United to continue to decline, of course they will rise again to can compete again closely with other teams, including Manchester City, which is no longer in doubt at the moment.
Maybe it will take time to recover and find his best performance and I hope Ten Hag can make Manchester United spread its wings again. Ten Hag must be able to solve the problems that occurred with Manchester United which made them play less than optimally. As you said, Ten Hag has to improve his defense, and we'll just have to wait and see what Manchester United will look like.