This is ridiculous. This would make me kill someone if true. But it's probably not, since it makes no sense. Any transfer rumour should be considered on 3 fronts before even contemplating if it's true, and those are:
i. Does the move make sense for the player?
ii. Does the move make sense for the selling team?
iii. Does the move make sense for the buying team?
Now, this doesn't satisfy point 3, because Dortmund don't want to sell, but they might have to, so let's ignore that one. It doesn't make much sense for point 1. Sure, Lewandowski wants to leave for a bigger team in a bigger league, but seriously, ManU? For at least the next 2 seasons, assuming no one gets sold, he'd have Rooney and RVP in front of him, in a team that sometimes only plays one striker, with Chicharito and Welbeck breathing down his neck, as well as the form of the wingers and attacking mids like Young, Nani, Valencia and Kagawa who, depending on formation, would also be competing with him. He's a starter at a top club in a top league and there are other teams who would love to have him and who could offer him actual playing time (like, any other team barring perhaps City and Barca, even Madrid, with their Higuains and Benzemas have less competition in the ST role than ManU).
And point 2 is tied to the one I just discussed. Seriously? You JUST bought RVP. Rooney still has many years in front of him, and they're both arguably (quite definitely) better than Lewandowski right now. There's also Chicharito who has 7 in the last 5, and Welbeck who... I dunno, but he's definitely too good to be 5th choice. This is all makes me think that this is just lazy journalism, but if it is, why not link him to Chelsea? Or Spurs? Or even Arsenal? This makes so little sense, that I'm starting to think it has to be true. Why else would it pop up? After all, City did something similar when they got rich and exclusively bought strikers (consider that this summer they had Dzeko, Tevez, Balotelli, Aguero, Adebayor, and Santa Cruz on their books, and that's AFTER selling at least 3 other forwards who were bought in th last few seasons).
Bascally I'm just astonished that this even a rumor and deathly scared that it might even be true. I want me some Lewandowski in blue. I wanted me some RVP in blue and some Chicharito in blue in the summer. So **** off you bastards. Buy a ******* midfielder (but not one that we might want).
" and they were like "cool, we'll replace him with Kompany" 
