The 34-year-old would have earned $18 million if his option had been exercised.
The Rangers acquired Kluber from the Cleveland Indians in December with the hopes he would be able to stabilize their rotation behind Mike Minor. He was coming off an injury-plagued 2019 in which he posted a 5.80 ERA and 1.654 WHIP over seven starts.
However, his 2020 season was over almost as soon as it got started, when he was removed from his first start on July 26 after one inning with tightness in his right shoulder.
The Rangers announced Kluber was being shut down for four weeks with a Grade 2 tear of the teres major muscle in his right shoulder. He didn't have to undergo surgery, but the shortened 60-game season meant he wouldn't have enough time to get back on the mound.
When he was at his best from 2014-18, he was one of the most dominant pitchers in Major League Baseball. The Alabama native won two Cy Young Awards in 2014 and 2017 and made three straight All-Star teams from 2016-18, posting a 2.85 ERA in 160 starts during that span.
Given his age and recent injury history, Kluber seemed likely to become a free agent. He will likely receive short-term offers with a lot of incentives for teams to protect themselves in case he gets hurt again, but the upside is huge if he can regain that previous form.