Complex sentence: His first professional experience came with UD Las Palmas on loan, in 2006--07's second division; during his first two campaigns with Real Sociedad's main squad he was an undisputed first-choice, also in the second level.
Split into simpler sentences: His first professional experience came on loan, for UD Las Palmas, in 2006--07's second division. During his first two campaigns with Real Sociedad's main squad, he was an undisputed first-choice, also in the second level.