"We want to get some of that muscle strength back," Mills said.
There is still no timetable for Lowrie's return. After Lowrie sprained his right ankle and injured the peroneal nerve in his right leg on July 14, the team estimated that he would need four to six weeks to heal. He's been on the disabled list since July 15.
Lowrie was batting .253 with 14 homers and 36 RBIs in 285 at-bats before the injury. Marwin Gonzalez has been the regular shortstop in Lowrie's absence.
Lowrie isn't the only Astros regular rehabilitating an injury.
Catcher Jason Castro, on the disabled list since July 8 with right knee effusion and currently on a rehab assignment, caught five innings on Sunday for Double-A Corpus Christi, going 1-for-1 with two walks.
Mills said Castro traveled to Triple-A Oklahoma City on Monday and will catch seven innings in a couple of games. Provided Castro doesn't encounter any pain in the knee, the team will have him catch nine innings in back-to-back games before considering bringing him back to the Major League club.
Castro was batting .254 with two homers and 20 RBIs on the season. He hit .323 with a .400 on-base percentage in his last 11 games before the injury.