Complex sentence: Riga was unregistered and re-registered with the club in August 2009 so he could be given the squad number 10 and Ivan Klasnić could take his previous number, 17, the season adter he was changed from number 10 to number 15.
Split into simpler sentences: Riga was unregistered and re-registered with the club in August 2009 so he could be given the squad number 10 and Ivan Klasnić could take his previous number, 17. The season after he was changed from number 10 to number 15.