On April 8, 2015, Nathan was placed on the 15-day disabled list due to a strained right elbow. During a rehab start with the Toledo Mud Hens on April 22, Nathan re-injured his elbow after throwing only 10 pitches. The same night, Nathan underwent MRIs, which tested positive revealing tears in his ulnar collateral ligament of the elbow and his pronator teres muscle, and would undergo Tommy John surgery, ending Nathan's 2015 season. Sources projected that this surgery could end Nathan's career, but he was not planning to retire yet.