The team announced Monday that Kluber underwent an MRI, which revealed a Grade 2 tear of the teres major muscle in the back of his right shoulder. The club said he will be shut down from throwing for at least four weeks and will move to the injured list.
"It's a blow, there is no getting around it," Rangers President and general manager Jon Daniels told reporters Monday. "Felt like he was in a really good spot, had no physical issues at all. In talking to him today, he feels better than he has in years physically. No indication that this was coming. He was throwing the ball really well."
Daniels said the right-hander will receive a platelet-rich plasma injection this week. Left-hander Kolby Allard will replace Kluber in the Rangers' starting rotation.
"Given the time frame here and the nature of a 60-game schedule and everything that goes with it, the timeline is not on our side," Daniels said. "We will give him his period to rest, and then if it's recovered, or the injury is healed to the point where he can begin throwing, he'll do so with the potential to possibly be ready for the end of the season.
"If that is the case, it'd probably be more likely as a reliever, given the time needed to build up as a starter."
Kluber exited Sunday's game against the Colorado Rockies after one scoreless inning due to tightness behind the shoulder. It was his first MLB start since May 2019, when he broke his right forearm on a line drive to the mound. He also dealt with an oblique injury during his rehabilitation last year.
The Rangers acquired Kluber in a trade with the Cleveland Indians in December. Before his injury last season, he was the AL Cy Young winner in 2014 and 2017 and posted 20 wins in 2018.