That's exactly what they said about Castro last year... all through winter, ST, and even throughout April. Yet, Castro made his debut in early May... forcing Theriot to 2nd and the DeWitt/Baker experiment to the bench after he tore up the Southern League in April. If Jackson does the same thing next year, he'll get the call too.
Plus, I wonder how reliable a guy that works for the front office of the Arizona team (that has few real ties to the front office of the MLB team... and assuming this is true... remember, minor league teams generally have a front office staff of fewer than 5 people: team owner, a GM -which is usually the owner-, a forward scout, a marketing exec -often the team owner-, and a secretary to answer phone calls) can be when there's no real GM at the MLB level, and a large potential for the current manager/coaching staff at the MLB level to be gone in a few weeks.
I think the only way Jackson isn't in the opening day lineup next year is if he has a terrible show with Team USA and a lukewarm ST... or if they manage to bring in someone like Cuddyer to play RF.