Unlike the previous season, when David De Gea was still Man United's goalkeeper, the club did not concede as many goals as last season. If Erik Ten Hag had not replaced him with Andre Onana, the club would not have experienced as many problems.
Blaming Onana completely is actually also bad. I think there are many factors when Man United concede a lot this season. You have to remember this season there are not many Man United defenders in good shape. Many Man United players are injured. So it was purely in the team not Onana. Ten Hag's strategy also has a lot of blunders. The players made a lot of mistakes. The flow of the ball from below does not go well, possession is easily lost. When under tight pressure, Man United is always weak to protect the ball. Many players are confused about where the ball will be given. It's a classic problem but Ten Hag has to work hard to eliminate these mistakes.