Keepers usually have a preference or are better behind either a good defense or a bad one. For example Courtois isn't the type to single handedly save a side being battered but is very astute at keeping concentration to make only 2 saves a game and do nothing the rest, i would put Oblak in that category aswell. Then you have the keepers like DDG and prime Cassilas who can virtually take on and keep out opposition attacks all by themselves and get their team's points despite the other 10 players looking like schoolchildren but lack/lacked the concentration to make that one save after having a quiet hour or so. Courtois is struggling because playing infront of a weak defense as Real currently are isn't his game, put Oblak in and he would struggle aswell.
It's very rare you find a keeper who aces both those aspects, probably only Ter Stegan out of the keepers today does.
Casillas was exceptional in the 2010 WC playing behind a team that barely gave the opposition a sniff. All great keepers are great at doing both. De Gea, Neuer and plenty of others have excelled in both, Courtois hasn't because he's not a top keeper. That's not to say the de Gea's performances for Spain doesn't raise question marks but the characterisation of his time at United and Atletico as being a constant fire fighter isn't true. In fact his two worst seasons, this one and his first, were the two seasons where he's been forced to make the most saves per game.
Under van Gaal, when he was consistently our best player, he made roughly as many saves as Courtois. Watching him every week it was pretty obvious that we limited the opposition to relatively few chances, but the chances we conceded were particularly dangerous (conceding possession in the defensive third or being caught out in behind) and he was often exceptional doing relatively little. Look where we were in the shots conceded table in
LvG's first season. Rock bottom. That's when he was starting to be talked about as the best in the world by United fans. The next year we were 3rd bottom. Fast forward to this season where we've faced more shots than 11 other teams in the league...and he's been undeniably below par.
That's not to say de Gea can't excel as a firefighter, he can and he has, but the correlation is the exact opposite of the one portrayed in the current narrative because of that one tournament. Courtois' struggles are much less complicated.