He was brilliant the season before last, excellent at the world cup & now he is very good at the Copa.
Was last season's slump because of Manchester, Van Gaal or Angel himself?
I would say that all three things played a part in di Maria's performances last season. At the start, van Gaal gave him total freedom to play as a left winger/wide central midfielder, and he flourished in such a role. When van Gaal changed to a 4-3-3 formation, though, di Maria played a more restricted winger role whilst not getting much support from the midfielders, which made it tougher for him to perform well. This was when we had Fellaini in the side and wanted to take advantage of his aerial strengths. It was during this period when he faced more physical challenges from opposing markers, and he struggled to deal with them. His confidence dipped as he was beaten by his marker more often, and his performances dipped as a result. I wouldn't really say that van Gaal was the primary cause of di Maria's dip in performances in this period as di Maria himself had yet to adjust to the physical nature of the Premier League, and he was bound to face more of these physical challenges anyways once other teams got to know him better.
After he came back from his November injuries, van Gaal tried to fit him into a role where he was given all of the attacking freedom and no defensive responsibility. Seeing that di Maria struggled with the physical challenges from opponents, he tried to put di Maria into a role where losing the ball wouldn't hurt us that much. This was when he played up front for us in the 3-1-4-2, then as an attacking midfielder in a diamond, and then on the right wing similar to his Benfica, Real Madrid (under Mourinho), and Argentina winger roles. During this time, di Maria's confidence was low, and that showed in his inconsistent performances (moments of brilliance + several chances created followed by poor deliveries, losing the ball easily, and moments of frustration).
I believe di Maria's performances last season were more down to his adjustment to the Premier League and a lack of confidence resulting from that adjustment. All of this culminated in him being benched, and during this time, his confidence was shot down even more. I doubt that he's faced this many physical challenges ever in Portugal and Spain, and I doubt that he was ever benched for a couple of months due to inconsistent performances outside of England. However, as we've seen in the Copa America, he's learned from his first season here, and he now deals with the physical challenges better. His confidence is back, and he's playing at his best once again. After adjusting to the league in the first season, I believe that di Maria will be a much better player for us in the second season, and I'm sure that he'll be more of a consistent threat than he was in the first season.