Mariano Rivera made the splitter the pitch that it is.
Yes, but Jack Morris (my favorite-ever Tigers-tosser) made the splitter formidable and changed the “fast-ball” into a set-up pitch for both his splitter and the circle-change.
Morris, for all his bluster and “pagentry” while with the Tigers, could throw both those pitches with accuracy. If Jack hit someone...well...he meant to hit them. That’s why I like Morris.