I've been taking some interest in how young talent develops when players reach the major leagues. Some of that is because the Phils have called up Dom Brown.
Some random data points, this is by no means any sort of exhaustive study --
Miguel Cabrera, 1st 25 games, 100 PA, 4 HR, .222/.290/.456, OPS of .746
Chase Utley, 1st 25 games, 102 PA, 5 HR, .235/.265/.449, OPS of .714
Ryan Howard, 1st 25 games of 2005, 90 PA, 5 HR, .238/.311/.475, OPS of .786
So you can't really say any of those guys came up and lit it up right away. But then I looked at Albert Pujols and here's an example of a guy who probably made for no doubt that he was going to be a great one.
107 PA, 8 HR, .365/.430/.719, an OPS of 1.149. Amazing.