HOUSTON -- With right-hander Lucas Harrell leading all National League rookies in innings pitched with 133 1/3 entering play Monday, Astros manager Brad Mills said the club might have to start monitoring his innings soon.

Harrell, who's 6-1 with a 2.02 ERA in nine home starts, pitched a career-high 161 2/3 innings in 2010 between the White Sox and Triple-A Charlotte. He threw 146 in the Minors for the White Sox in 2009, and then added another 10 in the IBAF Baseball World Cup.

"We're definitely going to have to watch that moving forward a little bit," Mills said.

Mills didn't rule out using a six-man rotation when rosters expanded at the beginning of September, which would have more to do with limiting innings for everyone in the rotation than just Harrell. As far as Harrell is concerned, Mills said there's a possibility they could skip him in the rotation at some point down the road.

Mills emphasized the team hasn't had in-depth discussions about limiting Harrell's workload.

Lowrie, Castro progressing from injuries

HOUSTON -- Wearing a new knee brace since the middle of last week, shortstop Jed Lowrie has better range of motion in the knee, according to Astros manager Brad Mills.

"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.