Even from the replays been shown, it wasn't even a mistake from the part of Manchester United, it wasn't a goalkeeper error. It's just that that Arsenal defender was smart enough to disrupt the goalkeeper, that would have been an infringement on the goalkeeper and I wonder why the ref didn't cancel the goal even after his seen the replay. Although, Manchester United players were not clinical enough if not, they would have drawn or even won the match.
Some people considered it a foul because an Arsenal player touched the goalkeeper, but the referee made the decision and the goal was allowed to stand. The defenders performed well in the match, with almost no major errors. Admittedly Arsenal were much luckier creating chances, though they didn't capitalize on many. The manager was satisfied with Manchester United's performance despite the defeat, and their play has improved slightly.
At least the performance of the Manchester United players has improved slightly, and we can see the changes being made. The next match is against Fulham, and I see this as an opportunity for Sekso to show his quality as a player with an instinct for scoring goals, and hopefully Amorim will give him a starting spot.