Complex sentence: January 4,2016 A nagging knee injury prevented him from officiating much of the season, Crawford returned during March 2016 but on March 10,2016 Crawford announced he would be retiring from NBA officiating effective immediately, due to his knee not healing as expected.
Split into simpler sentences: January 4,2016 Due to a nagging knee injury preventing him from officiating much of the season, Crawford returned during March 2016, but decided to retire immediately due to an inevitable knee injury. On March 10,2016, Crawford announced he would be retiring from NBA officiating effective immediately, due to his knee not healing as expected.