born in germany, mastelić moved back at the age of 2 with his mother to the family's native brnaze, near sinj, in croatia, and he started training football there, at the lower-tier . a versatile player, starting out as a midfielder, but playing at center-back, winger and a forward, he moved, at the age of 12 and at the behest of coach slaven lakić, to the second-tier junak sinj. he became a first-team regular, now more firmly a forward or a winger, at the age of 16, amassing 20 caps, with a single goal. that did not, however, go unnoticed, and he was snapped up by nearby giants hnk hajduk split, where he joined the academy.