The 33-year-old, who was a key reliever in 2016 and 2017, returned to the Rangers on a minor league deal in December and had been recovering from surgery to repair a partially torn ulnar collateral ligament in his right elbow.
In 2018, Bush attempted to become a starter but was moved to the bullpen midway through the season. He was then demoted to Triple-A Round Rock to work on his command when the injury occurred.
In nine appearances with Frisco, Bush has allowed only three hits in 8.2 innings of work and has struck out seven. Bush's fastball sat at 95 mph and topped out at 97, according to Jared Sandler.