Onana is good goalkeeper only when he was playing at Inter Milan and Ajax but for Manchester United Onana performance is dissapointing and he is too often to make blunder which sometimes makes Manchester United failed to beat their opponents but until now Amorim never discuss about new goalkeeper as the replacement of Onana so i assume Onana position as Manchester United first goalkeeper is still safe and as far i know Amorim currently more focus to look for new left back or new striker because Amorim feel these positions is the weaknesses of Manchester United
Yes, despite that Onana's performance since he came to Manchester United dropped and hasn't met the level of performances when at these two clubs you made mention, Ajax and inter Milan, but this season alone ,Onana form has taken a new shape, and there is indeed an improvement to his performance and we need to recognize that. Manchester united problem is not just left in the hands of the keeper but it a general issue, if Manchester United team picked form as supposed, then you would notice how Incredible that Onana will be this season.
Generally, his performance has been fair this season.