Whilst André Onana is a decent goalkeeper, i do not think that he is a good upgrade to de Gea, if Manchester United are going to let de Gea leave, i expect them to replace him with someone who is far better than he is, and i don't think Onana is that guy. De Gea has been quite exceptional for Manchester United all these years, but i think it is high time he left the club, in the last couple of years he has become famous for making really costly mistakes, thus for both parties it is better he leaves now and take up a new challenge elsewhere.