They did made some bad decisions like did not sign a new contract with David De Gea, spent massive money to buy Onana, did not spend enough money for a central defender position to improve the heart area of their defense system.
De Gea is a good goalkeeper in saving skills and he loves Manchester United after many years there. If Manchester United and Ten Hag were more cleverly, they would have been kept De Gea in their club and use him as an alternative goalkeeper for Onana. I believe De Gea will accept that position as so far he did not join any club.
Onana is over rated because he has better passing skills than De Gea but in general, combining all skills of a goalkeeper, I think he is not a better goalkeeper than De Gea. Hopefully Ten Hag will not have to pay an expensive cost by his coach position because of his belief in Onana.
Manchester United had De Gea but let him off to chase Onana because of his good leg work whereas judging from all areas, De Gea was as keeper with better chances of saving the team from so many unnecessary goals. No doubt that Onana is good but De Gea would have done.
De Gea has this quality of being United's savior in their most critical times and it's now occured to them now that Onana wasn't as good as how much he was rated, they just have to maintain Onana as their goalie hoping that in crucial times, he'd deliver positively. Onana did a lot better with his last team, no one actually knows now what the problem with him is but he sure would make good saves as games come by.