Onana is a good goalkeeper, but I agree that his performances are not so consistent that it makes Manchester United's defense easy to penetrate sometimes. To bring in a new goalkeeper, of course they have to sell their main goalkeeper, unless they will make one of them as a second cover. However, usually when a team brings in a new goalkeeper, they have to sacrifice their main goalkeeper to be sold or loaned. But this is also not something that can be guaranteed to be successful, I mean sometimes a change does not bring anything better. The point is they have to be very selective when they want to bring in new players.
Whatever it means, Manchester United goalkeeper is improving in all matches, he's no robot but a human being that's not above imperfections. He makes mistakes and apologized, the team pardons his acts and also exhibits potentials that served him to be in great form, atleast in the red jerseys. André Onana have both a great goalkeeper and a bad one for Manchester United since his arrival at Old Trafford. There are some games, where the Cameroonian makes crucial mistakes that would definitely lead to goals and there are important matches, he stands up as the promising savior for the Red Devils.
Ruben Amorim wouldn't accept the ideology of signing a new goalkeeper because he has more compressing issues at hand, his squad in not balance and in January transfer window, he will simply make efforts to appoints the specialized targeted players he pleases. There'd something to noticed about Amorim, he's working so hard to get Manchester United back on track because it's never an easy task task to handled the managerial role in Old Trafford and in one of these elite teams where the expectations is driven high by the fans and the team's management. Watch and see if he's potent to elevate the club's level of performance this season or wait till the next.