I think he will.
It will be an epic comeback if De Gea return to Manchester United, especially if he can prove if Manchester United was wrong by making many saves. A goalkeeper can still able to play in top league up to 40 years old e.g. Buffon, Neuer, and Casillas.
Onana will be kicked out after De Gea signed, because Manchester United can just looking for potential goalkeeper rather than train Onana.
The current Manchester United coach is a person who has different ideas. I highly doubt he would bring in the former Manchester United goalkeeper. Knowing Amorin, he will bring in some young goalkeepers and young players who are not very famous and will make them play well so that they become popular and famous, and then they will be sold for a high price.
The Manchester coach will want to create a team of talented young players that the club has not spent a lot of money on to buy them, and then after they win titles they will sell them for high prices. Just like he did at Sporting.