A product of Navarran side CA Osasuna's youth system, Astiz's earlier career was played with the B-team. This was where he met manager Jan Urban, a former Osasuna Polish player, who was team manager at the time. Thus, he spent 2007-08 on loan to Legia Warsaw (where Urban would serve as manager) in the Polish Orange Ekstraklasa, thus becoming the first Spaniard ever to play in that competition.
In June 2008, Astiz returned to Osasuna, only to return immediately to Legia on a five-year deal, despite still having a contract running. Osasuna, however, retained the possibility to re-buy the player after the first two seasons.