He can't because he doesn't have the work permit to do so. Man U is playing in the Champions League match against Celtic this week and Chelsea Sunday and its extremely unlikely that he'd get any time in those matches even if he had the proper paperwork. He's eligible to hit the transfer market on June 2nd. Middle of the MLS season but just after most Euro clubs conclude their seasons. It'll interesting to see if he plays for DC United and then transfers or just waits until he signs somewhere and then plays in the fall. Regardless, the MLS should make a pretty penny off the bidding war between Chelsea, Manchester United, Real Madrid, Barcelona, etc.