De Gea saved LvG much more than he has saved Mourinho, backed by De Gea's back to back PotY awards. He didn't win it last year. I don't think he'll win it this year either.
Under LvG in that second season, it felt like it was every single game De Gea was relied on. This season, I'd wager that the games where we have been truly reliant on out-of-this-world saves made by De Gea would be in single figures, and I struggle to remember a game last season where his heroics won it for us on a regular basis. Last night he made one very good reactive save to push it over, but it was also right at him. He would have had to dive out of the way of it for it to go in. Good reactions to paw it to safety, but he's made better saves in his career. It would be poor goalkeeping if it goes in, because it was a very poor finish.
The defence certainly needs looking at, but the centre back pairing was purely circumstantial. We played the only two who were fit. If everyone was fit, would Smalling and Lindelof be the starting pair? Bailly would be one of them for certain, and the other place is well up for grabs. Rojo was very good pre ACL injury, and Bailly has been out for a long time, two things Mourinho can't really be blamed for. Smalling is dependable, and Jones is very good when fit, it's too early to write of Lindelof but I'm not too sure about him yet. There are good individuals in there, it's just about making a first choice pair that works, and keeping that pair fit. Mourinho seems to love a centre back pair that plays every game together, but it's something he's not been able to do here, and that no manager has been able to do at United since about 2008.
I don't really see an issue overall. He's arguably the best goalkeeper in the world. I'm fine with him being relied on. If you have the best goalscorer in the world, you rely on him to score goals. Best shot stopper is relied on to stop shots. Whilst we're relying on him a bit more this season than last season, it's still nowhere near the level it was in LvG's second season, particularly from December onwards.