On August 23, 2008, just a year after turning pro, Casimero defeated Rodel Quilaton for the then vacant Philippines Boxing Federeration (PBF) flyweight title, winning a unanimous decision (UD) through ten rounds. Following the bout, Casimero moved down one division, to light flyweight, defeating Liempetch Sor Veerapol of Thailand by technical knockout (TKO) in the fifth round to claim the then vacant WBO Asia Pacific Light Flyweight title. Casimero successfully defended that title two bouts later against Ardin Diale, until he got the biggest break of his career.
