If you build on realism, then when the alien is transparent, you will not distinguish the moment of impact and you will not be able to parry it at all. If a predator attacks in a state of visibility, then you will not be able to pair it, well, try for example to repulse a blow of a sledgehammer with a knife. Knife fight in reality - it is possible to fend off when the opponent delivers a cutting biting blow with the same quick blow. If a quick blow is delivered by something more severe, then an attempt to repel it with a knife will result in either a hand injury or the knife will fly out of it, or if you apply force to hold the knife, then you will not have enough parry speed.
So I think parry is complete nonsense and game needs some else mechanics, but not parrying with a tiny knife, cuz it’s looks really weird. Just remember first two movies about predator - nobody could parry nothing (Billy, King Willy, Jerry).