I think the goalkeeper and defense line both have shortcomings, in the sense that they still need a series of adaptations. This is Onana first season, and we know that most of Man United defenders have just returned from injury, ultimately causing United to concede in every game they play. It's true that Ten Hag must quickly find a solution for United XI formation, and I don't think there's any harm in giving time to young players, if the more senior players consistently make United performance plummet.
Exactly, if Ten Hag cannot bring United to consistently achieve points, the consequences will be very bad, he will not qualify for the last 16 of the UCL, and in the EPL his position will continue to sink, and will end his tenure at Old Trafford sooner.
If ten hag fails to qualify for the round of sixteen in the UCL they he should be considered a failure, it may be very true that he made good progress in his first season at old Trafford but it doesn't mean he is permitted to fail now because he has had an achievement already which was qualify for the UCL and he was hail for achieving such a mile stone in his first season.
Onana does not need to whole season to be good at his craft, when degea came to Manchester united he kept several clean sheets in his first season and there are other records too which has been kept by other players in their first season at a club, so it pains me when people excuse onana with the fact that he needs time and it's a good reason to fail.
The defense line of Manchester united is still weak and I think it's more in the duty of the coach to see to fixing that, most of the defenders suffered injury but they are back and should be strong enough by now but the sad part is that these boys lack enough communication amongst them selves to deliver and at this point the keeper and the coach should be blamed because they are the once watching this defender closely enough to correct them but they seem to have failed at it.