Munchkin Booty: is there a time limit for "Not so fast me bucko"
I can't seem to find anything about the definition of immediately in the official FAQ. But my interpretation would be that as long as no other actions were taken between the drawing of monster and the playing of the card, that would still count as immediate.

But as the rules themselves say


  Any disputes should be settled by loud arguments, with the owner of the game having the final word.