Didn't John spend some time with Columbus last year, too? VanAllen I think is close to ready. I think JZ may need some more time this year with Harrisburg and Columbus. Thing is, if they aren't absolutely ready, and get crushed at their first ML appearance, you could destroy what confidence they had and that can take time to rebuild. That's for the ML coaches to evaluate and determine - how ready any given player is mentally. I'm sure the minor league coaches and managers have a hand in their development and promotions as well. They are interviewed about how quickly pitcher X handles a loss or a bad inning. Right now Brownlie in Columbus was on the fast track, that is until his last appearance - he got SLAAAAMMMMMED!!!! Got rocked for 5 runs /4 hits in one inning. His ERA for that game was 45.00. Future scouts will look to his next game to see how he bounces back. So, every player is different and has a different path to the Bigs. If you see good numbers but no promotion to the Bigs, you can bet that player has been eveluated as not-ready emotionally/mentally.
Remember, "90% of the game is mental - the other half is physical."