Blaming Onana for the second goal actually in my opinion is too imposing and seems ridiculous because it just wants to give a mistake to him considering Onana often makes blunders but for this match it was too much when Onana was blamed.
There are some reasons that I think are forced if it's still blaming Onana. The second goal came with a quick counter-attack that was difficult to anticipate. Indeed in this case Onana was not in his coverage area and did not seem to want to force himself to move but I think that would not have happened if there was proper guarding for Iwobi because we know that with the fast counter-attack that was done actually Iwobi was without really good guarding which made him very easily able to score in this match while silencing Manchester United with a score of 1-2.
Onana in this match made saves that were actually very good and I think that is quite commendable, it's just that it seems like Ten Hag always tries some players like Mainoo who is used as a defensive midfielder even though he is very good as a second striker.
The loss of Hojlund is also very impactful in terms of Manchester United's attack because his finishing is still compromised.
Onana's criticism from fans is not his fault, but a result of the general team's performance. Onana before now in his previous teams was as good as ever, he won awards and made good name for himself. His move to Manchester United and his performance afterwards surprised everyone because suddenly, he began to concede consistently and it was disappointing for the club as a whole. Do not blame Onana for his flops, Manchester United right now needs reformation, their defense, midfield, attack and team management as a whole needs to be fixed.
Manchester United has suffered series of set backs in both team management and match outcomes, sometimes it's the manager's fault, sometimes the individual players, sometimes it's everyone's fault at thesame time. But I'd say that the manager is to be blamed fully because he's the overseer of the team and he has the capacity to make corrections when things go wrong so no matter how much a player is blamed, you won't tell me that the manager didn't also see the defaults in that player and make a move for his replacement. United's squad from back to forth are world class and I believe they just need the right hand. Let's use Onana as an instance, the major reason why he was signed by United was a result of his top notch performance with Ajax and now here's what they get. They need a new strategy, they also need a new managerial atmosphere, maybe that's where the jinx lies. Hopefully, they'd work on themselves and bring back the fighting spirit Onana experienced in his last teams.