You answered it yourself. The ability is always there, but after experiencing several goalkeeping coaches, two negative-minded managers, and unreliable back four, he needs time to adjust. Whether Ten Haag has the patience or De Gea could improve drastically, it remains to be seen.
I think Ten Haag will assess De Gea's situation at the end of this season. I will not be surprised if Ten Haag decides to activate the one-year extension in De Gea's contract if he manages to perform at an acceptable level. A great goalkeeper is not easy to find. I will be wary of bringing a flavor of the day like Diogo Costa. He just made the stupidest mistake for someone considered a top goalkeeper. Robert Sanchez is a safer bet. I'm also interested in Vanja Milinkovic-Savic, but not as a guaranteed #1 goalkeeper.