John Blake, the Rangers' executive vice president of communications, announced Kluber was experiencing tightness in the back of his right shoulder. He'll be evaluated further on Monday.
The man nicknamed "Klubot" played for the Cleveland Indians from 2011-2019, winning the American League Cy Young award twice. He paced the league with a 2.25 ERA and 0.87 WHIP in 2017 and struck out an average of 246 batters per year from 2014-2018.
Kluber also won 20 games for the first time in his career in 2018.
He was off to an unusually slow start in 2019 after going 2-3 with a 5.80 ERA over seven starts. Per FanGraphs, his walk rate jumped from 4 percent to 8.9 percent, and he allowed a career-high 37.5 percent hard hit rate.
Unfortunately, Kluber's 2019 season ended on May 1 after he suffered a fractured right arm against the Miami Marlins when a Brian Anderson line drive struck him.
He made a few rehab starts in the minor leagues but was shut down for the year after suffering abdominal tightness during his third minor league start on August 18.
Kluber was traded to the Rangers on Dec. 18 for Delino DeShields and Emmanuel Clase.
Joe Palumbo replaced Kluber on Sunday and figures to be one of the potential replacements for the right-hander should he have to miss a start. Kolby Allard, who made nine starts in the majors in 2019, will be another option as well.