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.
I don't think all these players mentioned have same issue, the case of Morata and Lukaku are the same but different from that of Onana. Morata and Lukaku have this problem of not providing goals for the club, and the expectations was high and it made these two players to perform very poorly. They don't have it at heart to play for the club as of then, it was too much.
Onana is a goalkeeper that Manchester United fan wanted him to do the duty of a goalkeeper and a defender too, which is not suppose to be so.
David de Gea never had this kind of pressure, he carried the duty of a goalkeeper and a defender on his shoulder. So if they want the best of Onana I think everybody should try as much as they can to do well in their respective positions, especially the defense, that alone will make Onana do well to protect the goalpost.
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.
That's another negative effect they need to deal with when they get back from the break and I think it will break the players mentally at least for some time just like the case of Juventus. If Man UTD don't take advantage of this Everton's current situation I feel the game might end in a draw and if it happens that this encounter end in a draw, that would motivate the Everton side to step up to win the next game.