From the rulebook:
BACKUP (X): Take the top X cards from the Zombie Discard Pile ...
REINFORCEMENT: Draw 1 card from the Zombie Deck, put it face up on (Graveyard Area) in this Field . If it is an Event, resolve it and then destroy it...
Backup draws from the discard pile, not the draw deck.
Reinforcement draws from the deck, if you draw the Boss event card, the boss will appear at that point.
So sorry, you need to prepare the goblins, because the boss will come, sooner or later.