De Gea is still quite good and even for now during his return to Serie A his performance is very good but of course it is also not a reasonable thing if Manchester United are looking for him again as an option for Onana at this time.
De Gea is already comfortable with his position at Fiorentina so in this case even if Manchester United want him back I think he will also think several times about agreeing to this.
Manchester United made mistakes in the past and re-signing De Gea is a step of redemption in my opinion. But I agree in another view because re-signing De Gea could not be so good because Manchester United have made a bad decision and inadvertently hurt his feelings. In terms of quality he is still pretty good and his average game at Fiorentina is quite impressive.
It is better to look for other options by bringing in a young goalkeeper who has quality so that in the long term they have a much better goalkeeper. Onana does need to be given a little pressure because with another goalkeeper there will be competition for a regular place at the club.
The current situation actually only has to focus on professionalism, I think because however these two parties are a little difficult if they are forced together it is not too possible.
De Gea's age must be taken into consideration even though we can see that his performance is very good at Fiorentina but his current age is not too young anymore even though this does not have a big effect because he is a goalkeeper but still Serie A and the EPL are clearly different so it would be better if De Gea stays in his current place where Fiortentina has considered him very good and Manchester United must look for other options to replace Onana's current role.
Indeed, cooperation is still possible if both parties agree to this but of course with the awkward situation a few seasons ago I think it will be very difficult if in the end both parties (Manchester United and De Gea) can be fine later when this possibility occurs.