On 9 September 1984, one month shy of his 18th birthday, he made his first-team – and La Liga – debut, in a 1–0 home loss against Málaga,[2] as a central defender, the position in which he started his career;[3] this was due to a strike by the professional footballers in the country, with most sides having to resort to fielding youth players.

[4] After only four league games over his first two seasons, Loren became an undisputed starter for the side from Gipuzkoa, helping them to the 1987 Copa del Rey – he did not play in the final against Atlético Madrid due to injury, however.

In the 1987–88 campaign, by now reinvented as a striker by Welsh manager John Toshack, he scored a career-best 11 goals in 36 games as the club finished second to Real Madrid.

[5][6][7] In the summer of 1989, Loren signed for Real Sociedad's local rivals Athletic Bilbao, who paid 300 million pesetas for his services, a record for a national player.

[14][15] Again under Toshack, he returned to the stopper position, often partnering another youth graduate, José Antonio Pikabea, in his second spell, and never appearing in less than 27 league games in the following seven seasons.