I am glued to this stuff (United fan like yourself) and the Falcao signing completely shocked me. I don't think United will keep him after his season long loan is up but I think we do need him. RVP requires an operation he has put off for far too long (for the injury sustained in the 3-0 win against Olympiakos) so he could play in the World Cup. He's looked badly off his game since the first couple of matches in the group stages and often can't finish the full 90 minutes anyway. Falcao is a similar player and RVP's operation could count him out for over 2 months. Besides, he's injury prone anyway and getting older and slower.
United ins:
Shaw, good signing for the long term. The new LB for years to come and his age means that transfer fee will look more like value a few seasons down the line.
Herrera, a skilled midfielder, the kind of player United need since they have such a lack of quality in midfield. Depending on the arrival of Strootman he could be put down to the bench in January but United have lacked depth in midfield.
Rojo, not a bad price for his age and skill. Had a good World Cup and once his work permit is sorted he will be a good CB for United. He can also be LB cover for Shaw.
Di Maria, the most expensive PL player and a great signing. However, the formation doesn't play to his strengths because it lacks wingers. Despite this and his high price, he is a truly exceptional player, something United lacked last season.
Blind, can play at CM, CB or LB and worth his fee. Intelligent and skilled, he will be played as a defensive midfielder and will excel. Might also play centre back depending on how Phil Jones does.
Falcao, there to replace RVP for a season. I think he's here because Wilson isn't quite ready yet. His arrival will displace some players but he should provide a boost for the season.
All these players coming in means the squad will change, we haven't seen United and the new signings in action properly yet because of work permits or injuries but that will change and some issues will be solved. However, the new players will take some time to gel with each other. Without injuries I'd play 4-2-3-1 of De Gea, Rafael, Jones, Rojo, Shaw, Herrera, Rooney (yes as a CM), Januzaj, Mata, Di Maria, Falcao. Subs: Lindegaard, Blind, Smalling, Carrick, Fellaini, Wilson, RVP.
As for your picks of Hummels and Vidal, Hummels isn't coming mate, he never was. He wants to stay at Dortmund, his name was linked with United because he'd be perfect for us but there's nothing to it and I wish there was. As for Vidal, his agent is the one who has been stirring things up. True that United need a great midfielder but it's best to save that fee for Strootman. Besides, if you notice my formation I'd play Rooney in the role Vidal would have taken, Rooney has proven he can score from midfield and he gets lots of assists too. Since we've got a lot of strikers and only a few slots he should be put somewhere his lack of pace doesn't hamper the team too much.