I seriously disagree.
First of all, context counts. The reality of the matter is that in recent seasons, United has not been a great place for a young player to develop. Dortmund was. He made a wise choice, and a difficult one - United would have been a lot closer to home and probably would have paid him more money. He should be applauded for his maturity, not seen as someone who told us to "feck off".
Besides, even if it was a case of a young, arrogant teenager telling us to feck off - would you really hold a decision an 18-year-old player made for the rest of his career?
As for signing players from Dortmund, I certainly understand the need to be wary considering our recent signings from there. But it just means we have to be a lot wiser with our scouting and planning. Ruling out someone right away because he plays for Dortmund is just not smart.