Complex sentence: After two seasons backing up Urruti, Arconada became the club's undisputed starter, being instrumental in back-to-back La Liga titles, which led--after the second--to a semifinal run in the European Cup.
Split into simpler sentences: After two seasons backing up Urruti, Arconada became the club's undisputed starter. He was instrumental in back-to-back La Liga titles, which led--after the second--to a semifinal run in the European Cup.