I don’t see the need to bring De Gea back to Manchester United. The decision to part ways has already been made, United have to move on. If Onana is no longer m good enough for the team they can sort other options elsewhere, I believe De Gea is not the only good keeper available to Manchester United.
For me in this case bringing back De Gea will only make the situation awkward.
De Gea has felt how he was not appreciated before and Manchester United should also now look for an alternative because if in the end when something like this happens where they bring De Gea back and the situation does not change it will only be the same thing as a few seasons ago.
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.