Well, I imagine either the notion where teams don't have a book on someone is either A) untrue, just a thing announcers and fans say or B) pro scouting departments resources are actually thin enough that they are not looking at single A and AA players with an eye towards supporting the major league staff, just with an eye towards trades. Nobody could have expected Haseley to be a starting CF in the majors this season.
A lot of teams don't even have pro scouts anymore, it's all video!
I think Haseley can handle this, but we still don't even know if he'll be in the majors by August 1. Even if they don't acquire an OF, he is technically filling Sean Rodriguez's spot (while Williams is filling Bruce's spot). But if he does well (and Franco is continuing to play) keeping him up would be great too.