Klein was called up Wednesday from Triple-A Round Rock and pitched against the Boston Red Sox. Texas designated for assignment outfielder Carlos Peguero and sent right-hander Stolmy Pimentel to Round Rock.
In his previous 23 major league games, all with Texas, Klein was 1-2 with a 3.80 ERA. He began this season with the Rangers and had a 7.71 ERA without a decision in six games before being optioned April 14.
Peguero hit .186 with four homers and nine RBIs in 30 games for the Rangers this year. Pimentel was 0-1 with a 3.97 ERA in eight relief appearance with Texas.