it may be a piece of his blade but that is the same scene posted above and i checked he has 4 swords that form his Buster Sword, they may be called different and counted different as their assembled but there are only 4. when his main sword is knocked out of his hand this is the one he uses in defense.

you never explained the Aeris Zack, Cloud Tifa issue who's hooked up and not and how and why.