Time to move on - I don't think you can have a 'keeper that every time he takes the field you half expect him to give a cheap goal away, that can't be easy for the players in front of him either and must make them hesitant in their decision-making as well and impact their performances.
Not sure how much United would get for him if they sold him in the summer if this continues but it would be worth taking a punt on another GK if they can get Onana out and another in and not spend any money that probably has and should be earmarked for other positions. Maybe somebody like Hermansen - not sure if he is good enough either but at least you are swapping a player who clearly isn't for one who might be without any financial impact.
Between him, Antony and Martinez and the £175m or so spent on these as well it does make you somewhat question whether there should really be greater resistance to new manager's coming in and signing 'their' player so-to-speak. Managers come and go and the club need to prioritize talent and good value deals over familiarity surely?
Hindsight maybe but when you have Vicario moving to Spurs in the same window for 2.5x less money and he wasn't exactly under the radar given he'd come off a great season in Serie A that year as well, it seems like paying the premium for the player a manager has worked with before often backfires, typically if they are stepping up a level (Onana to be fair wasn't given he went to Inter between Ajax and United, but I think there were always question marks about his error rate).