Complex sentence: Despite a great deal of financial turmoil during that time, he turned the struggling team around to the extent that they set a club record of 21 games unbeaten in the league during season 1988--89.
Split into simpler sentences: Despite a great deal of financial turmoil during that time, he turned the struggling team around. This improvement included a club record of 21 games unbeaten in the league during season 1988--89.