i'd disagree - the options to sacrifice a shield or take a level of fatigue to negate a critical hit are listed in generalist sections (sacrifice shield in the shields section of equipment, take fatigue under...well...fatigue). monsters can wield shields and, presumably, take fatigue, and considering sacrifice shield is written as if it is an inherent ability of shields, and taking fatigue to negate a critical hit is written as if it is an inherent property of fatigue, i think it's reasonable to say a monster could do either. after all, monsters can dash and dodge and disengage and whatnot, and those things are listed in the adventurer's guide and not the monstrous managerie. now, my point isn't that your proposed ruling isn't also reasonable - i could absolutely see a DM making a ruling that monsters can't negate critical hits by taking fatigue or sacrificing a shield - just that, when evaluating the balance of furious critical, i don't believe we can reliably assume it to hold in any given game.