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 red @-@ 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 .