Affeldt struggled with injury and inconsistency in 2015. By late June, he had a 5.96 ERA and allowed five home runs, more than he allowed from 2012 to 2014 combined. He was placed on the disabled list with a left shoulder strain on June 26. Affeldt returned July 22, but on August 24, he suffered another off-the-field injury, a left knee subluxation, while spending time at a lake with his family on an off-day. On October 1, 2015, Affeldt announced he would retire after the 2015 season.