Truthfully, I think that United and Meneer Van Gaal just need to re-work their formation, we all can see that Rooney is not the pure striker we knew before, over the course of four games we saw Wazza running back to collect the ball, back passed the ball to the Schmidfield and wasn't in the box when the crosses came. United need to pull Wayne back and put him in Second Striker position or a CAM and let Chicharito be the pure striker. Watch the Club Brugge match and see for yourself, the moment Chicha stepped in, the play became lively, Chicha became a genuine threat inside the box and if Chicha can't reach the high cross, we have Big Felli to score with the header. With Manchester United overstock with midfielders, I can see Fellaini as a striker, pouncing those crosses from Mata and Memphis, much like his goal against Brugge. I know I am not the manager, Mr. Van Gaal but I think you should open your mind to a good suggestion.
Nevertheless, if United are still keen to sign another World-Class Striker, I believe they only have two possible options. Number one is Thomas Muller, now this one is rather impossible but there might be some wiggle room though, the problem is Bayern already sold Basti to United and they do not want to lose Thomas either, as far as my concern, I think United have a better shot with Mario Gotze than Thomas Muller. Even if United can get Thomas Muller with an insane amount of money, I don't think they will get him this season.
The most likely option is Gareth Bale, let me tell you why. With Madrid signing Mateo Kovacic, Madrid now have too many attacking midfielders to support Cristiano Ronaldo and with Madrid fans being Madrid fans, booing their fans to the Bernabeu exit, I do believe Manchester United have an excellent chance Gareth Bale with a caveat, of course. Man Utd need to act fast and sign Bale before Arsenal sign Karim Benzema because is Arsenal beat United then I'm pretty sure, Madrid won't let Bale go. Throw David De Gea in the deal, it would fasten the process and the rest will uncover themselves.