Complex sentence: Kyle, who had been Sunderland's manager since 1905, resigned at the end of the 1928-29 season, after 25 seasons in charge of the club, managing 817 matches and taking Sunderland to the verge of a League and cup double.
Split into simpler sentences: Kyle, who had been Sunderland's manager since 1905, resigned at the end of the 1928-29 season. He had amassed a total of 25 seasons in charge of the club, managing 817 matches and taking Sunderland to the verge of a League and cup double.