You can make the same argument for European players:
Taibi - Our worst keeper
Cryuff - Flop, failed to impress after a solid Euros
Ronaldo - Wanted to leave for Spain
De Gea - Ditto
Berbatov - Like Veron, flashes of brilliance but couldn't fit into team
Saha - Injury prone
Tosic - Never good enough
Zaha - Poor attitude
Bebe - lol
Should we stop buying European players?
Good point. I think its all down to selecting players who were either the wrong choices to begin with, as in several of the European choices above, or in the cases where we did make appropriate selections, they may have not had the temperament to be long term United players. Di Maria, Tevez, and Heinze were the correct players in terms of talent and skill, but were unfortunately mercenaries who were more interested in making money by bouncing between clubs than by staying in one place.