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.
André Onana is a very decent and good goalie that I think is absolutely good enough to replace David De Gea at Manchester United. If Erik Ten Hag can get the services of Onana, it'll surely be a good transfer deal for the club.
For Pep Guardiola to praise the goalkeeper and talk about how extraordinary he is before and after their UEFA Champions League final game tells how great of a replacement he'll be to De Gea at United