I think Erik Ten Hag already has a potential replacement for David De Gea if he leaves, I think Andre Onana will be the candidate if he will replace De Gea,
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.
Even though de Gea is a great goalkeeper for Manchester United, but if de Gea's performance has decreased then of course Manchester United should look for a replacement. But regarding André Onana, I think André Onana will still stay at Inter in the next one or two seasons because after all, André Onana is still under contract at Inter until 2027 and currently André Onana is just in his first season at Inter. Thus, it is too soon or maybe it is not right if André Onana is immediately linked to replace de Gea in the near future. Besides, I think that Manchester United's defense is weak because maybe the defenders are still not quite right, because for Manchester United's goalkeepers they often concede goals due to the poor performance of their defenders.