What a horrible start for Onana, to think he was one of the best goalkeepers in Europe last year.
The hype is what has gotten you disappointed so far. Before he moved to Manchester United, there was plenty hype with the news, but what do the fans really want? for him not to concede even with Manchester Uniteds.poor defence? he is not super man. Onana is just a good keeper with extra passing ability which sometimes becomes a problem also for him, a little wrong decision making in his position which is always a critical one can put his team behind.
No matter how good a goalkeeper is, If the defense of the club is poor, it will affect the goalkeeper because there is no way, he will act as a goalkeeper and a defender all at once. He will only perform the best he can as a goalkeeper and the defense will defend and stop goals from entering the goalpost.
This is the exact issue Manchester United is facing currently. Their defense is already bad and they want Onana to conduct magic in the goalpost so that goals won't enter(Which is impossible).
One of the reasons Onana was signed into Manchester United is because he has modern goalkeeping skills in him, to give long and short passes without fear. Even though he's a quality goalkeeper.
The biggest problem that has occurred at Manchester United so far is the storm of injuries that has hit their players so that their performance has been quite chaotic at the start of this season. Moreover, there was news that their dressing room was not in a harmonious condition after there was news that Sancho and Erik had an argument on social media. Onana was not the cause of Manchester United defeat at the start of this season, but at least the picture of their game began to be in doubt at the start of this season.
I think De Gea can still be relied on by Manchester United this season but they didn't give him a contract extension which meant he had to leave the club he loves so much. For this season, I think Manchester United will find it difficult to compete with teams that have made better progress than them, such as Manchester City, Arsenal and Liverpool.