OK, here, perfect example. Troy Tulowitzki was drafted 7th overall last year as a SS out of LBSU, Longoria's alma mater. There has been quite a bit of comparison between Tulo, Longoria, and former LBSU SS and current A's SS Bobby Crosby. Tulo is considered by most scouts and executives to be the best of the three. Most think Tulo could step into a Major League role right now and put up better numbers than Bobby Crosby. Then Crosby, then Longoria. Longoria is not far behind Crosby in terms of where he was when he get out of school. With no rush to get Longoria to the show, it is probable he'll follow a path similar to those guys he is most often compared with - Tulo and Crosby.
Troy signed a little late last year and played at Modesto, Colorado's A+ affiliate. He opened this season at AA.
Crosby began his career at A+ Modesto (then the A's affiliate) and finished that season there, hitting .395 in a limited amount of time. He began his first full season of pro ball BACK at A+ and pretty much tore the league up for half a season. It took HALF A SEASON for him to get promoted to AA, and he finished the year there. He moved up to AAA to begin his second full year, received a September callup and has been in the pros ever since.
Because Longoria signed so early, they started him at SS ball. They may promote him to A+ soon, but he will finish the season at A+ unless he absolutely tears the cover off the ball. Promoting players too soon completely impedes their development, and I don't think the Rays are going to take that chance on this guy.