Manchester United conceded 14 goals in six matches is not entirely the goalkeeper's responsibility, there are defenders who also have a role in maintaining the defense well, but unfortunately, United's defense is very weak when Varane or Luke Shaw are not playing.
It is impossible for Ten Hag to be on the bench for Andre Onana because replacing him with Manchester United's second goalkeeper might make things worse, what United is currently facing is not a problem in goalkeeping but in all areas, and that is quite unfortunate after last season's performance was better, but since the start of the season they have looked weaker than last season, and losing to Galasataray in the champions league added to the length of their defeat at Old Trafford this season at least they have experienced this 3 times so appearing in front of their fans is not enough to help them perform better, including later against Copenhagen who can provide tough resistance to Bayern Munich, and seeing Manchester United still at the bottom of the rankings is quite sad.
Of these 14 goals, at least two were entirely on Onan's conscience - this is a lot for a distance of 6 games. If we talk about the other goals, it is clear that this is the result of team play, but so far Onana is not “saving” at the last line. He doesn't do anything special, after all he is playing worse so far than De Gea who had bad days (sometimes) but sometimes he saved United and it was only thanks to him that United got points.
I think by the way that there is nothing wrong with giving the reserve goalkeeper a chance, if I remember correctly, in the days of Ferguson, after unsuccessful games, the reserve goalkeeper got his chance and the main goalkeeper knew that he must match the level if he wants to always play in the first team.