If you have a good memory, I want you to think about the first time you walked into a new school, or even just your freshman year in High School. The nervousness of not really knowing what was awaiting you ahead.
Sure, you had been to school before, but not this...not High School. You went from the high man on the totem pole to the lowest in a matter of days. You were unsure about everything you once knew so well. Where at one time everything was so comfortable, now you found yourself almost second-guessing yourself.
I would image this is the same situation for a player who was drafted from NCAA football into the NFL.