The injury will be helpful for Onana to have a rest and time to find a psychology balance that is very necessary for him and his career in Manchester United. He has been playing under a lot of pressure and eyes that seem to take him down whenever he makes mistakes and give them chances to attack. Without an injury, it's hard for Onana to have a necessary break time as Ten Hag still considers Onana as a best goalkeeper in Manchester United.
There is rumor that Manchester United and Ten Hag want to bring David De Gea back in the winter transfer season at least with a short term contract. The injury of Onana when it's about one month till the open time of transfer market will give them more reasons to think about this idea more carefully and seriously.
Is not the end of the world for Manchester United and don't say it like Onana is the only goalkeeper they have in that team. I believe with him being out of action it would give him (Onana) time to rest and it will reduce the amount of pressure on him. The second choice could cover for Onana and even do better, these days goalkeepers are the ones who look for opportunity to showcase themselves just to be in the first team.
Playing under huge pressure and after having bad start is not easy for all player. Pressure is very harmful for player performance and if they fail to be strong against pressure, their performances will be affected negatively. We saw it with Morata, Lukaku in Chelsea as without confidence, a player will play worse than what he can do with his best. Therefore, a few weeks to sit back and take a break, avoid more pressure whilst have time to analyze past mistakes will help Onana to improve a lot when he returns.
Well as for the coach Ten Hag, he's the only one who knows how to make his team get a win in their next game against Everton, with Onana out of action, the absence of the Cameroon goalkeeper won't tell if they win or not, is just left for the coach and the team to make it happen.
Everton just got a shock with 10 points deduction because of their financial issues. They are in a most vulnerable time that gives Manchester United a very good opportunity to beat Everton in the next match. It's hard to explain if Manchester United can not complete this task with three points.