Rojo is a very interesting case.
He arrived at Sporting with no credit. Out of favour at Spartak Moscow. Sporting was a sinking ship in 12/13 and he looked awful. Got sent off in the big games. Completely out of sorts. Amazingly enough... he was still getting called up for the Argentina NT and playing out of position. He was a centre back at Sporting and a left back for the NT. 13/14 was a much better year for Sporting and for Rojo as well. Much more solid and composed, but still... he was sent off 3 times. He wasn't tested in Europe since Sporting had failed to qualify for Europe the previous season. Even among Sporting fans the opinion about him only changed over the last few months, especially with the World Cup where he did well.
I don't know what to believe, the Rojo centre back who was always getting sent off against Porto and Benfica or the Argentina left back. One things is for sure, buying a playing for big money based on a few games is not a very good idea. He is not United material(haha, as in, for a team that wants to be among the top of Europe), it's a big jump in the dark. I suppose he is a good option for the wingback position in that 3-5-2 formation, because as a centre back... he is very very dodgy.
I think United are being betrayed by that early success in pre-season and with Van Gaal wanting to show his 'authority' or 'identity' by giving every player a chance. Things went well and there wasn't much urgency in signing people. Just look at Herrera and Shaw - not Van Gaal's targets, he gave the ok and United splashed out ridiculous amounts of money for them. So now, the house is on fire, the romance is over and they must sign players and must ditch other ones as soon as possible.
Di Maria was very very inconsistent throughout his career(only had a top season at Benfica, before being sold). At Madrid he was always a bit erratic - his decision making was typically very very poor. Last season he reivented himself. From his traditional winger position, he moved back on the pitch and he was really great in many games. So fast and running all game, he was capable of pressuring people, recovering balls and carrying the ball down the picth at great speed. He was vital for Madrid last season, giving them a great injection of physicality in the middle of the pitch(with Alonso and Modrid they were soft). Thing is, what position will he play at United? And how will he handle being the 'main star'? It is one thing playing second fiddle to Ronaldo, it is a different one having to be the main man.
I would imagine that if Van Gaal sticks to his silly system, Di Maria would be like a Robben, but unlike Holland, who seemingly were happy being boring and playing counter attack, United will have to create much more and boss games and there won't be much space for a runner like him. Then there is the issue of Mata, Rooney and Van Persie. I think ideally, Van Gaal, ditches his system and adopts a more conventional lineup and almost inevitably with Mata starting from a wide position and cutting inside, he will have freedom, but on paper he will be a winger. This is the only option because Di Maria should start on the wing and then Rooney probably roaming behind Van Persie as a sort of number 10.