De Gea is the best shot stopper in the world. However, it simply isn't enough. If you want to be a top team these days, you need to play with a high line, press high and be very good in and out of possession. In order to achive this, you need a goalkeeper who is proactive and dominant off his line, whilst being good on the ball. Truly the eleventh man, rather than 10 outfield players and a keeper. De Gea is far too passive and definitely a problem.
Despite all of this, there are areas where we simply don't have any options. For example, all of our wide players are better on the left. Sancho is perfectly capable on the right, but I'd rather he features in his optimum position on the left. Elanga can do a job on the right, but is better on the left. He's also a kid and shouldn't be playing as often as he is. Rashford is abysmal wherever he plays these days and Greenwood is out of the picture. I would have preferred to see him used as a centre forward regardless.
Other than Matic, we don't have a defensive midfielder. Up top we have a declining Ronaldo and a half-arsed Cavani, who is leaving in the summer. The other irritating thing is just when you are led to believe that a certain position has been filled for the foreseeable future, individual performances decline massively. I'm thinking of Shaw, Maguire and Wan-Bissaka. You can argue that some, or all of those players were never good enough to begin with, but I think that Maguire and to a lesser extent Wan-Bissaka, were reasonably competent up until this season. Shaw was playing at a world class level last season. These guys were intented to be mainstays in our side, but in two of those cases they have lost out to players who aren't good enough themselves, in Telles and Dalot.
It will be interesting to see where the new manager chooses to prioritise. Obviously he will assess our current options, but I suppose it also partly comes down to his own stylistic preferences and how integral a specific set of attributes are to the playing style/philosophy he would like to implement.