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.