I agree that fans need to lower their expectations for Onana's appearance. He will be fine this season if he gets good support, but if he gets pressure then it certainly won't help him change. Ten Hag must take a good approach to improve the form of each of his players, this includes improving the dressing room atmosphere to be more harmonious.
Perhaps because every expensive player is expected to meet the fans' high expectations, it creates unique pressures for them. De Gea was acquired by Man United for 13 million, while Onana was brought in for over 50 million. From this, fans hope that Onana can outperform De Gea. And we all don't know what kind of pressure haunts expensive players when they struggle to adapt and are forced to perform at their best. You are right; it does take some time for Onana to adjust to the EPL.
What I find significant is the lack of creativity in Man United's defense. Ten Hag's tactics haven't found the right synergy among the players, so the reason for Man United's poor start to the season could be the absence of a strong chemistry between new and old players.
Having just acquired Hojlund, why is Ten Hag rumored to be searching for a new striker again?