It wasnt an official trial. He came to train with the United squad thats all (officially) but anyways he would have to go back as he was still contracted to the MSL and still too young to be signed. A lot of people need to remember the non-parallel season calendars of the MLS with almost any other league. He came to the club to train and stay in shape (much as Eddie Johnson with Reading, Josh Wolff with Everton, as well as a host of other mls-based players have done in the past, in-between seasons). Obviously, the club can and does take advantage of this by having a look at whomever is there; but it's no different than when dwight yorke trained with the club pre-WC.
