The main reason de gea left old trafford was none other than ten hag who did not like de gea style of play so far which could not build the game from behind line, so he was no longer a priority for the Manchester squad even though he was actually willing to cut his salary to stay at Manchester United at that time, Manchester United decision to bring in Onana had made his position even more difficult before deciding to leave
With that terrible pass, and other mistakes previously, Onana is a bad goalkeeper and your screenshot shows it too. David De Gea with his return in Fiorentina, shows that he is still one of best goalkeepers globally and surely Onana can not compare with De Gea in performances. In many stats, De Gea is better than Onana, and Ten Hag was sacked because he failed at Manchester United. Many transfer failures, Onana, Antony, Casemiro and more, there is no reason to defend Ten Hag that he was right with decision on De Gea and Onana, and other decisions in the club as well.
even previously de gea was without a club for a year before joining fiorentina earlier this season, his decision to join Fiorentina is the right move at this time, because De Gea has rediscovered his best performance, with his solid performance has provided stability for the fiorentina defence which so far is one of the 3 teams that conceded the least in serie A.
In Fiorentina, De Gea made many excellent saves for them and contributes to their high rank in Serie A now. His great saves for Fiorentina reminds me about his golden time in Manchester United when he was considered as 50% of their power.