I will actually say that if they have to get a goalkeeper De Gea is going to be a great option for them. But they should not expect anything long term from him. Probably 2 years of good service is expected from him at the maximum. But on the other hand, I also agree that they should get a defender first. Defense is a very big problem for them at this moment. And with also injuries being a big problem, defense is and should be the main priority for them right now to fix. The amount of goals that they are conceding is definitely not the fault of the goalkeeper in my opinion.
De Gea returning to Manchester United? Only if Manchester United are shameless after expelling De Gea and then want him back now.
Although indeed something like this could happen again but of course De Gea must remember the previous humiliation he received from Manchester United which when he was still preparing to make a contract extension even though his salary was reduced but Manchester United seemed to turn a blind eye and did not want to see De Gea return because they wanted Onana more.
With a situation like this, I actually think something like that is quite difficult for Manchester United even though it can still be done unless De Gea wants it even though he has received previous humiliation.