Cashner, who is 3-6 with a 3.50 ERA in 12 starts, said he felt the discomfort in the second inning of his last start against the Astros on Wednesday. He underwent an MRI on Friday.
"I knew when it happened that I tweaked something, but you never want to come out of a game as long as you can throw," Cashner said. "It's unfortunate but it's not going to be too long."
Cashner was scheduled to start on Tuesday against the Blue Jays. Nick Martinez will likely take that start but he is in the bullpen this weekend as the long reliever.
The Rangers called up reliever Ernesto Frieri from Triple-A Round Rock. The Rangers signed him to a Minor League contract just 11 days ago after the Yankees designated him for assignment. He had been pitching at Triple-A for them and made one appearance for Round Rock before being called up.
Frieri, 31, made a combined 18 appearances for the two teams and was 2-0 with a 2.86 ERA. In 22 innings, he has allowed 14 hits and nine walks while striking out 26.
Frieri was the Angels' closer in 2012-14 when he had a combined 71 saves. But he started having trouble giving up home runs. He allowed 1.89 home runs per nine innings in 2013-15, the second highest for a Major League pitcher with at least 100 innings.
"The biggest report I got was from a scout from another organization when we first got him," Banister said. "He still has good stuff, a good guy and he competes. Still has a good fastball and you can see his secondary stuff is in good shape…similar to what we have seen in the past. If he gets it in the strike zone, it makes it challenging for a hitter."
To make room on the 40-man roster, the Rangers designated Triple-A outfielder Peter O'Brien for assignment. He was claimed on waivers from the Reds on May 27.
T.R. Sullivan has covered the Rangers since 1989, and for MLB.com since 2006. Follow him on Twitter @Sullivan_Ranger and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.