Complex sentence: Immediately after the Derby, Oath was rated an "average" Derby winner, with a rating of 123, but his injury meant that unlike most Derby winners, he never had the opportunity to improve on this assessment.
Split into simpler sentences: Immediately after the Derby, Oath was rated an "average" Derby winner, with a rating of 123. His injury meant that unlike many Derby winners, he never had the opportunity to improve on this assessment.