Ideally, with any automatice transmission you should not start the vehicle up in the morning and just put it in gear. Sometimes it takes time for the hydraulic circuits to fill and purge all the air out of the system.
I would suggest that you give the car 5 or 10 seconds or so before putting the car into gear. This is also good for your engine as most engine wear occurs at cold start up when there is the lowest amount of lubrication. If you give the car a few seconds before putting into gear, you let the oil fully circulate and allow there to be a better protective film before the engine gets loaded (put into gear).
2012 Ingot Silver Mustang - 3.7L V6, Performance Package (traded)
2012 HP White Mustang GT - 5.0L, Premium, 3.73 rear end