Looking at this first goal, it's easy to blame De Gea. Sure it's a shot he should and most of the time would save, but like any keeper this kind of blunder does happen. The point is what's going on before this goal. Ronaldo receives the ball surrounded by Brentford players. He's struggling to maintain possession; Bruno who was busy directing his teammates as the throw-in was coming in actually RUNS AWAY from Ronaldo, the player who was behind Bruno is now free to intercept the ball which he does, winning possession and then passing to Dasilva.
Ronaldo struggling to hold onto the ball running on to it with a player infront and behind him, and Fred and Bruno decide the winning move is to run away from him to a position that he cannot see.
Fred as a holding midfielder doing that and leaving that much space is absolutely criminal. Or perhaps they've been coached to do that: if Ronaldo drops deep, they're to push up to stop the opposition back-line from collapsing on them and boxing them in. Although the fact Fred was subbed off at halftime would indicate otherwise.
Pause the video when DaSilva receives the ball. There's no united player within 15 yards of him and he's within shooting range. That's a mistake that will get punished more times than any team can afford.
These kinds of issues are nothing to do with the keeper, you swap him for the most inform goalie in the world and you're still gonna have these problems. Keane alluded to Bruno's attitude a while ago implying he was buying into his own hype and doing too much talking. That a player has that much room to shoot 10 minutes into a game is the real problem.
From the moment DaSilva receives the ball you can see he's got one thing in mind. At no point does he lift his eyes away from the ball, he's gonna shoot, you can see it a mile away. Yet Martinelli makes a lacklustre attempt to close him down, he should be flying at him fully committed, in the end it's a half-hearted tackle, Dasilva gets the shot off and scores.
DeGea held his nerve for years but sometimes when you see so much obvious stuff going wrong right in front of you it's hard to keep your spirit applied.
Edit: Also as the shot's going in there's a player sprinting into the box, if De Gea spills it, he gets to it first because Maguire who's stood right behind him and can see what he's doing doesn't react with nearly the same commitment. It's just too easy to blame one person for this. This is a collective effort.