Of course he's still young and relatively inexperienced but the 'worrying' thing is that both the late Arsenal goal and the Newcastle (where his defender miskicked the ball) was 'simple' positioning errors.
Both were near post finishes. Against Arsenal, I don't think it was a bad decision to come out and block the shot but he shouldn't have launched himself (i.e get airbourne) at that shot. You need to close the space, stay grounded and keep your feet together to block anything along the floor cause there's no way any other shot could have gone in. Against Newcastle, his feet was too slow to shuffle across and again close the space.
I've admitedly not watched many games so now I am obviously going to make an unjust comparison here...but It's a bit Foster/Pickford/Hart/Robinson-esque in that he has very good shot stopping ability and reactions but too gung ho with the 'saving' aspect and neglecting protecting his angles etc. The issue is that this is not something you can readily learn unless you have a great goalkeeping coach/manager nurturing the fundamentals.
On the one hand, I wouldn't mind if he came back to Utd as the undisputed no.2 and get to learn/compete with De Gea. It's one of those 'you don't know until you do', so perhaps he would thrive on the pressure and up his game even more. Maybe his (elite?) confidence can help him here. Sometimes it's about your approach and mentality as much as your base skillset..
Then on the other, it certainly wouldn't hurt for him to have another year out on loan.