It is only a training session. At least that is what is being claimed in the latest Dallas Morning News:
Johnson, 20, is underway with a two-week spell at Manchester United. From there, it's on to PSV, which already employs fast-rising American winger DaMarcus Beasley. The Dutch Eredivisie leader also is close to securing a spot in Champions League knockout stages.
These training spells aren't official tryouts; obtaining a work permit would be unlikely for now.